Migrate to `Uri.normalizePath()` and `Iterable.empty`.

[pq]

We could cleanup packagemap.dart and get away from defining our own stand-in:

static Uri _normalizePath(Uri existingUri) => new Uri().resolveUri(existingUri);

and migrate to using Uri.normalizePath().

If we do this, we'll need to up our SDK contsraint to be >=1.11.

[sethladd]

Go for it! packages.cfg won't have initial support until at least 1.11

[pq]

This is breaking observatory. Rolling back.

cc @hterkelsen

[harryterkelsen]

I think that the Uri.directory constructor also has to go. It's not in 1.10 https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/dart:core.Uri

[lrhn]

And we also rolled back the Iterable.empty use, which should also go back with 1.11.

[pq]

And we also rolled back the Iterable.empty use, which should also go back with 1.11.

3175bb9f34eb38d149cf98e9c1d82a901881e6e0

[pq]

Wow this has languished!

Fix up here: dart-archive/package_config#46.