Implementation of async-await transformation on js ast.

tests/language/asyncstar_yield_test.dart

+// 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.
+
+import "dart:async";
+import "package:expect/expect.dart";
+import "package:async_helper/async_helper.dart";
+
+Stream<int> foo1() async* {
+  yield 1;
+  var p = await new Future.value(10);
+  yield p + 10;
+}
+
+Stream<int> foo2() async* {
+  int i = 0;
+  while (true) {
+    await (new Future.delayed(new Duration(milliseconds: 0), () {}));
+    if (i > 10) return;
+    yield i;
+    i++;
+  }
+}
+
+Stream<int> foo3(p) async* {
+  int i = 0;
+  bool t = false;
+  yield null;
+  while (true) {
+    i++;
+    a: for (int i = 0; i < p; i++) {
+      if (!t) {
+        for (int j = 0; j < 3; j++) {
+          yield -1;
+          t = true;
+          break a;
+        }
+      }
+      await 4;
+      yield i;
+    }
+  }
+}
+
+Completer<bool> finalized = new Completer<bool>();
+
+Stream<int> foo4() async* {
+  int i = 0;
+  try {
+    while (true) {
+      yield i;
+      i++;
+    }
+  } finally {
+    // Canceling the stream-subscription should run the finalizer.
+    finalized.complete(true);
+  }
+}
+
+main () async {
+  asyncStart();
+  Expect.listEquals([1, 20], await (foo1().toList()));
+  Expect.listEquals([0, 1, 2, 3], await (foo2().take(4).toList()));
+  Expect.listEquals([null, -1, 0, 1, 2, 3, 0, 1, 2, 3],
+      await (foo3(4).take(10).toList()));
+  Expect.listEquals([0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
+      await (foo4().take(10).toList()));
+  Expect.isTrue(await (finalized.future));
+  asyncEnd();
+}