Implementation of async-await transformation on js ast.

tests/language/await_exceptions_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.
 
 // VMOptions=--optimization-counter-threshold=5
 
 import 'package:expect/expect.dart';
+import "package:async_helper/async_helper.dart";
 
 import 'dart:async';
 
 // It does not matter where a future is generated.
 bar(p) async => p;
 baz(p) => new Future(() => p);
 
 test0_1() async {
   throw 1;
 }
@@ skipping 44 matching lines @@
     }
   } catch (e) {
     x = e + 'd';
     return '?';
   } finally {
     return x;
   }
   return '!';
 }
 
-main() async {
+test() async {
   var result;
   for (int i = 0; i < 10; i++) {
     await test0();
     await test1();
     result = await test2();
     Expect.equals('abcd', result);
   }
+  await 1;
 }
+
+foo() {
+  throw "Error";
+}
+
+awaitFoo() async {
+  await foo();
+}
+
+main() {
+  asyncStart();
+  test().then((_) => awaitFoo().then(
+          (_) => Expect.fail("Should have thrown"),
+          onError: (error) => Expect.equals("Error", error)))
+      .whenComplete(asyncEnd);
+}