Add a workaround for the Rollup bug

Author: gaearon

packages/react-reconciler/src/ReactFiber.js

@@ -41,10 +41,12 @@ if (__DEV__) {
   var hasBadMapPolyfill = false;
   try {
     var nonExtensibleObject = Object.preventExtensions({});
-    /* eslint-disable no-new */
-    new Map([[nonExtensibleObject, null]]);
-    new Set([nonExtensibleObject]);
-    /* eslint-enable no-new */
+    var testMap = new Map([[nonExtensibleObject, null]]);
+    var testSet = new Set([nonExtensibleObject]);
+    // This is necessary for Rollup to not consider these unused.
+    // TODO: report this as a Rollup bug.
+    testMap.set(0, 0);
+    testSet.add(0);
   } catch (e) {
     // TODO: Consider warning about bad polyfills
     hasBadMapPolyfill = true;