Implementation of async-await transformation on js ast.

tests/language/syncstar_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 "package:expect/expect.dart";
+
+Iterable<int> foo1() sync* {
+  yield 1;
+}
+
+Iterable<int> foo2(p) sync* {
+  bool t = false;
+  yield null;
+  while (true) {
+    a: for (int i = 0; i < p; i++) {
+      if (!t) {
+        for (int j = 0; j < 3; j++) {
+          yield -1;
+          t = true;
+          break a;
+        }
+      }
+      yield i;
+    }
+  }
+}
+
+// p will be shared between all Iterators from the Iterable returned by foo3.
+// On the other hand each iterator will have its own i.
+Iterable<int> foo3(int p) sync* {
+  int i = 0;
+  i++;
+  p++;
+  yield p + i;
+}
+
+main() {
+  Expect.listEquals([1], foo1().toList());
+  Expect.listEquals([null, -1, 0, 1, 2, 3, 0, 1, 2, 3], foo2(4).take(10).toList());
+  Iterable t = foo3(0);
+  Iterator i1 = t.iterator;
+  Iterator i2 = t.iterator;
+  i1.moveNext();
+  i2.moveNext();
+  Expect.equals(2, i1.current);
+  Expect.equals(3, i2.current);
+}