Implementation of async-await transformation on js ast.

tests/language/await_for_cancel_test.dart

+// Copyright (c) 2014, 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";
+
+bool cancelled;
+
+test1() async {
+  cancelled = false;
+  try {
+    StreamController controller = infiniteStreamController();
+    outer: while(true) {
+      await for (var x in controller.stream) {
+        for (int j = 0; j < 10; j++) {
+          if (j == 5) break outer;
+        }
+      }
+    }
+  } finally {
+    Expect.isTrue(cancelled);
+  }
+}
+
+test2() async {
+  cancelled = false;
+  try {
+    StreamController controller = infiniteStreamController();
+    bool first = true;
+    outer: while(true) {
+      if (first) {
+        first = false;
+      } else {
+        break;
+      }
+      await for (var x in controller.stream) {
+        for (int j = 0; j < 10; j++) {
+          if (j == 5) continue outer;
+        }
+      }
+    }
+  } finally {
+    Expect.isTrue(cancelled);
+  }
+}
+
+main() async {
+  await test1();
+  await test2();
+
+}
+
+
+// Create a stream that produces numbers [1, 2, ... ]
+StreamController infiniteStreamController() {
+  StreamController controller;
+  Timer timer;
+  int counter = 0;
+
+  void tick(_) {
+    counter++;
+    controller.add(counter); // Ask stream to send counter values as event.
+  }
+
+  void startTimer() {
+    timer = new Timer.periodic(const Duration(milliseconds: 10), tick);
+  }
+
+  void stopTimer() {
+    if (timer != null) {
+      timer.cancel();
+      timer = null;
+    }
+  }
+
+  controller = new StreamController(
+      onListen: startTimer,
+      onPause: stopTimer,
+      onResume: startTimer,
+      onCancel: () {
+        cancelled = true;
+        stopTimer();
+      });
+
+  return controller;
+}