diff --git a/lib/packagemap.dart b/lib/packagemap.dart
new file mode 100644
index 0000000..46a1797
--- /dev/null
+++ b/lib/packagemap.dart
@@ -0,0 +1,232 @@
+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library package_config.packagemap;
+
+class Packages {
+  static const int _EQUALS = 0x3d;
+  static const int _CR = 0x0d;
+  static const int _NL = 0x0a;
+  static const int _NUMBER_SIGN = 0x23;
+
+  final Map<String, Uri> packageMapping;
+
+  Packages(this.packageMapping);
+
+  /// Resolves a URI to a non-package URI.
+  ///
+  /// If [uri] is a `package:` URI, the location is resolved wrt. the
+  /// [packageMapping].
+  /// Otherwise the original URI is returned.
+  Uri resolve(Uri uri) {
+    if (uri.scheme.toLowerCase() != "package") {
+      return uri;
+    }
+    if (uri.hasAuthority) {
+      throw new ArgumentError.value(uri, "uri", "Must not have authority");
+    }
+    if (uri.path.startsWith("/")) {
+      throw new ArgumentError.value(
+          uri, "uri", "Path must not start with '/'.");
+    }
+    // Normalizes the path by removing '.' and '..' segments.
+    uri = _normalizePath(uri);
+    String path = uri.path;
+    var slashIndex = path.indexOf('/');
+    String packageName;
+    String rest;
+    if (slashIndex < 0) {
+      packageName = path;
+      rest = "";
+    } else {
+      packageName = path.substring(0, slashIndex);
+      rest = path.substring(slashIndex + 1);
+    }
+    Uri packageLocation = packageMapping[packageName];
+    if (packageLocation == null) {
+      throw new ArgumentError.value(
+          uri, "uri", "Unknown package name: $packageName");
+    }
+    return packageLocation.resolveUri(new Uri(path: rest));
+  }
+
+  /// A stand in for uri.normalizePath(), coming in 1.11
+  static Uri _normalizePath(Uri existingUri) =>
+      new Uri().resolveUri(existingUri);
+
+  /// Parses a `packages.cfg` file into a `Packages` object.
+  ///
+  /// The [baseLocation] is used as a base URI to resolve all relative
+  /// URI references against.
+  ///
+  /// The `Packages` object allows resolving package: URIs and writing
+  /// the mapping back to a file or string.
+  /// The [packageMapping] will contain a simple mapping from package name
+  /// to package location.
+  static Packages parse(String source, Uri baseLocation) {
+    int index = 0;
+    Map<String, Uri> result = <String, Uri>{};
+    while (index < source.length) {
+      bool isComment = false;
+      int start = index;
+      int eqIndex = -1;
+      int end = source.length;
+      int char = source.codeUnitAt(index++);
+      if (char == _CR || char == _NL) {
+        continue;
+      }
+      if (char == _EQUALS) {
+        throw new FormatException("Missing package name", source, index - 1);
+      }
+      isComment = char == _NUMBER_SIGN;
+      while (index < source.length) {
+        char = source.codeUnitAt(index++);
+        if (char == _EQUALS && eqIndex < 0) {
+          eqIndex = index - 1;
+        } else if (char == _NL || char == _CR) {
+          end = index - 1;
+          break;
+        }
+      }
+      if (isComment) continue;
+      if (eqIndex < 0) {
+        throw new FormatException("No '=' on line", source, index - 1);
+      }
+      _checkIdentifier(source, start, eqIndex);
+      var packageName = source.substring(start, eqIndex);
+
+      var packageLocation = Uri.parse(source, eqIndex + 1, end);
+      if (!packageLocation.path.endsWith('/')) {
+        packageLocation =
+            packageLocation.replace(path: packageLocation.path + "/");
+      }
+      packageLocation = baseLocation.resolveUri(packageLocation);
+      if (result.containsKey(packageName)) {
+        throw new FormatException(
+            "Same package name occured twice.", source, start);
+      }
+      result[packageName] = packageLocation;
+    }
+    return new Packages(result);
+  }
+
+  /**
+   * Writes the mapping to a [StringSink].
+   *
+   * If [comment] is provided, the output will contain this comment
+   * with `#` in front of each line.
+   *
+   * If [baseUri] is provided, package locations will be made relative
+   * to the base URI, if possible, before writing.
+   */
+  void write(StringSink output, {Uri baseUri, String comment}) {
+    if (baseUri != null && !baseUri.isAbsolute) {
+      throw new ArgumentError.value(baseUri, "baseUri", "Must be absolute");
+    }
+
+    if (comment != null) {
+      for (var commentLine in comment.split('\n')) {
+        output.write('#');
+        output.writeln(commentLine);
+      }
+    } else {
+      output.write("# generated by package:packagecfg at ");
+      output.write(new DateTime.now());
+      output.writeln();
+    }
+
+    packageMapping.forEach((String packageName, Uri uri) {
+      // Validate packageName.
+      _checkIdentifier(packageName, 0, packageName.length);
+      output.write(packageName);
+
+      output.write('=');
+
+      // If baseUri provided, make uri relative.
+      if (baseUri != null) {
+        uri = relativize(uri, baseUri);
+      }
+      output.write(uri);
+      if (!uri.path.endsWith('/')) {
+        output.write('/');
+      }
+      output.writeln();
+    });
+  }
+
+  String toString() {
+    StringBuffer buffer = new StringBuffer();
+    write(buffer);
+    return buffer.toString();
+  }
+
+  static Uri relativize(Uri uri, Uri baseUri) {
+    if (uri.hasQuery || uri.hasFragment) {
+      uri = new Uri(
+          scheme: uri.scheme,
+          userInfo: uri.hasAuthority ? uri.userInfo : null,
+          host: uri.hasAuthority ? uri.host : null,
+          port: uri.hasAuthority ? uri.port : null,
+          path: uri.path);
+    }
+    if (!baseUri.isAbsolute) {
+      throw new ArgumentError("Base uri '$baseUri' must be absolute.");
+    }
+    // Already relative.
+    if (!uri.isAbsolute) return uri;
+
+    if (baseUri.scheme.toLowerCase() != uri.scheme.toLowerCase()) {
+      return uri;
+    }
+    // If authority differs, we could remove the scheme, but it's not worth it.
+    if (uri.hasAuthority != baseUri.hasAuthority) return uri;
+    if (uri.hasAuthority) {
+      if (uri.userInfo != baseUri.userInfo ||
+          uri.host.toLowerCase() != baseUri.host.toLowerCase() ||
+          uri.port != baseUri.port) {
+        return uri;
+      }
+    }
+
+    baseUri = _normalizePath(baseUri);
+    List<String> base = baseUri.pathSegments.toList();
+    if (base.isNotEmpty) {
+      base = new List<String>.from(base)..removeLast();
+    }
+    uri = _normalizePath(uri);
+    List<String> target = uri.pathSegments.toList();
+    int index = 0;
+    while (index < base.length && index < target.length) {
+      if (base[index] != target[index]) {
+        break;
+      }
+      index++;
+    }
+    if (index == base.length) {
+      return new Uri(path: target.skip(index).join('/'));
+    } else if (index > 0) {
+      return new Uri(
+          path: '../' * (base.length - index) + target.skip(index).join('/'));
+    } else {
+      return uri;
+    }
+  }
+
+  static bool _checkIdentifier(String string, int start, int end) {
+    const int a = 0x61;
+    const int z = 0x7a;
+    const int _ = 0x5f;
+    const int $ = 0x24;
+    if (start == end) return false;
+    for (int i = start; i < end; i++) {
+      var char = string.codeUnitAt(i);
+      if (char == _ || char == $) continue;
+      if ((char ^ 0x30) <= 9 && i > 0) continue;
+      char |= 0x20; // Lower-case letters.
+      if (char >= a && char <= z) continue;
+      throw new FormatException("Not an identifier", string, i);
+    }
+    return true;
+  }
+}
diff --git a/test/all.dart b/test/all.dart
index 76284af..795289c 100644
--- a/test/all.dart
+++ b/test/all.dart
@@ -2,8 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'package:test/test.dart';
+import 'test_packagemap.dart' as packagemap;
 
 main() {
-  // TODO: add tests!
+  packagemap.main();
 }
diff --git a/test/test_packagemap.dart b/test/test_packagemap.dart
new file mode 100644
index 0000000..bce20d4
--- /dev/null
+++ b/test/test_packagemap.dart
@@ -0,0 +1,149 @@
+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library test_all;
+
+import "package:package_config/packagemap.dart";
+import "package:test/test.dart";
+
+main() {
+  var base = Uri.parse("file:///one/two/three/packages.map");
+  test("empty", () {
+    var packages = Packages.parse(emptySample, base);
+    expect(packages.packageMapping, isEmpty);
+  });
+  test("comment only", () {
+    var packages = Packages.parse(commentOnlySample, base);
+    expect(packages.packageMapping, isEmpty);
+  });
+  test("empty lines only", () {
+    var packages = Packages.parse(emptyLinesSample, base);
+    expect(packages.packageMapping, isEmpty);
+  });
+
+  test("empty lines only", () {
+    var packages = Packages.parse(emptyLinesSample, base);
+    expect(packages.packageMapping, isEmpty);
+  });
+
+  test("single", () {
+    var packages = Packages.parse(singleRelativeSample, base);
+    expect(packages.packageMapping.keys.toList(), equals(["foo"]));
+    expect(packages.resolve(Uri.parse("package:foo/bar/baz.dart")),
+        equals(base.resolve("../test/").resolve("bar/baz.dart")));
+  });
+
+  test("single no slash", () {
+    var packages = Packages.parse(singleRelativeSampleNoSlash, base);
+    expect(packages.packageMapping.keys.toList(), equals(["foo"]));
+    expect(packages.resolve(Uri.parse("package:foo/bar/baz.dart")),
+        equals(base.resolve("../test/").resolve("bar/baz.dart")));
+  });
+
+  test("single no newline", () {
+    var packages = Packages.parse(singleRelativeSampleNoNewline, base);
+    expect(packages.packageMapping.keys.toList(), equals(["foo"]));
+    expect(packages.resolve(Uri.parse("package:foo/bar/baz.dart")),
+        equals(base.resolve("../test/").resolve("bar/baz.dart")));
+  });
+
+  test("single absolute", () {
+    var packages = Packages.parse(singleAbsoluteSample, base);
+    expect(packages.packageMapping.keys.toList(), equals(["foo"]));
+    expect(packages.resolve(Uri.parse("package:foo/bar/baz.dart")),
+        equals(Uri.parse("http://example.com/some/where/bar/baz.dart")));
+  });
+
+  test("multiple", () {
+    var packages = Packages.parse(multiRelativeSample, base);
+    expect(
+        packages.packageMapping.keys.toList()..sort(), equals(["bar", "foo"]));
+    expect(packages.resolve(Uri.parse("package:foo/bar/baz.dart")),
+        equals(base.resolve("../test/").resolve("bar/baz.dart")));
+    expect(packages.resolve(Uri.parse("package:bar/foo/baz.dart")),
+        equals(base.resolve("../test2/").resolve("foo/baz.dart")));
+  });
+
+  test("dot-dot 1", () {
+    var packages = Packages.parse(singleRelativeSample, base);
+    expect(packages.packageMapping.keys.toList(), equals(["foo"]));
+    expect(packages.resolve(Uri.parse("package:foo/qux/../bar/baz.dart")),
+        equals(base.resolve("../test/").resolve("bar/baz.dart")));
+  });
+
+  test("dot-dot 2", () {
+    var packages = Packages.parse(singleRelativeSample, base);
+    expect(packages.packageMapping.keys.toList(), equals(["foo"]));
+    expect(packages.resolve(Uri.parse("package:qux/../foo/bar/baz.dart")),
+        equals(base.resolve("../test/").resolve("bar/baz.dart")));
+  });
+
+  test("all valid chars", () {
+    var packages = Packages.parse(allValidCharsSample, base);
+    expect(packages.packageMapping.keys.toList(), equals([allValidChars]));
+    expect(packages.resolve(Uri.parse("package:$allValidChars/bar/baz.dart")),
+        equals(base.resolve("../test/").resolve("bar/baz.dart")));
+  });
+
+  test("no escapes", () {
+    expect(() => Packages.parse("x%41x=x", base), throws);
+  });
+
+  test("not identifiers", () {
+    expect(() => Packages.parse("1x=x", base), throws);
+    expect(() => Packages.parse(" x=x", base), throws);
+    expect(() => Packages.parse("\\x41x=x", base), throws);
+    expect(() => Packages.parse("x@x=x", base), throws);
+    expect(() => Packages.parse("x[x=x", base), throws);
+    expect(() => Packages.parse("x`x=x", base), throws);
+    expect(() => Packages.parse("x{x=x", base), throws);
+    expect(() => Packages.parse("x/x=x", base), throws);
+    expect(() => Packages.parse("x:x=x", base), throws);
+  });
+
+  test("same name twice", () {
+    expect(() => Packages.parse(singleRelativeSample * 2, base), throws);
+  });
+
+  for (String invalidSample in invalid) {
+    test("invalid '$invalidSample'", () {
+      var result;
+      try {
+        result = Packages.parse(invalidSample, base);
+      } on FormatException {
+        // expected
+        return;
+      }
+      fail("Resolved to $result");
+    });
+  }
+}
+
+// Valid samples.
+var emptySample = "";
+var commentOnlySample = "# comment only\n";
+var emptyLinesSample = "\n\n\r\n";
+var singleRelativeSample = "foo=../test/\n";
+var singleRelativeSampleNoSlash = "foo=../test\n";
+var singleRelativeSampleNoNewline = "foo=../test/";
+var singleAbsoluteSample = "foo=http://example.com/some/where/\n";
+var multiRelativeSample = "foo=../test/\nbar=../test2/\n";
+// All valid path segment characters in an URI.
+var allValidChars =
+    r"$0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz";
+var allValidCharsSample = "${allValidChars.replaceAll('=', '%3D')}=../test/\n";
+var allUnreservedChars =
+    "-.0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz~";
+
+// Invalid samples.
+var invalid = [
+  "foobar:baz.dart", // no equals
+  ".=../test/", // dot segment
+  "..=../test/", // dot-dot segment
+  "foo/bar=../test/", //
+  "/foo=../test/", // var multiSegmentSample
+  "?=../test/", // invalid characters in path segment.
+  "[=../test/", // invalid characters in path segment.
+  "x#=../test/", // invalid characters in path segment.
+];
diff --git a/tool/travis.sh b/tool/travis.sh
index 8fdabfd..8242381 100755
--- a/tool/travis.sh
+++ b/tool/travis.sh
@@ -9,6 +9,7 @@ set -e
 
 # Verify that the libraries are error free.
 dartanalyzer --fatal-warnings \
+  lib/packagemap.dart \
   test/all.dart
 
 # Run the tests.