Fix scope/context behavior in await transformer.

tests/language/await_future_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=--enable_async --optimization-counter-threshold=5
 
 import 'package:expect/expect.dart';
 
 import 'dart:async';
 
@@ skipping 30 matching lines @@
       throw x;
     } catch (e1) {
       var y = await baz(e1 + 1);
       throw y;
     }
   } catch (e2) {
     return e2;
   }
 }
 
+var result = "";
+
+reset() {
+  result = "";
+}
+
+observe(x) {
+  if (result.length > 0) {
+    result += ",";
+  }
+  result += x;
+}
+
+newTask(x) async => observe(x);
+
+awaitAll(tasks) async {
+  for (var task in tasks) {
+    // This actually opens up a new context level.
+    await task;
+  }
+}
+
+taskTest() async {
+  var tasks = [ newTask('a'), newTask('b'), newTask('c') ];
+  reset();
+  await awaitAll(tasks);
+  Expect.equals('a,b,c', result);
+}
+
 main() async {
   var result;
   for (int i = 0; i < 10; i++) {
     result = await foo();
     Expect.equals(result, 30);
     result = await quaz(17);
     Expect.equals(result, 17);
     result = await quazz();
     Expect.equals(result, 2);
+    await taskTest();
   }
 }