Implementation of async-await transformation on js ast.

tests/language/await_not_started_immediately_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:io';
+// Test that an async function does not start immediately.
 
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 
+var x = 0;
+
+foo() async {
+  x++;
+  await 1;
+  x++;
+}
+
 void main() {
-  if (Platform.isLinux) {
-    asyncStart();
-    var selfExe = new Link('/proc/self/exe');
-    Expect.isTrue(selfExe.targetSync().length > 0);
-    selfExe.target().then((target) {
-      Expect.isTrue(target.length > 0);
-      asyncEnd();
-    });
-  }
-}
+  asyncStart();
+  foo().then((_) => Expect.equals(2, x)).whenComplete(asyncEnd);
+  Expect.equals(0, x);
+}