Fix dart2js for switches in async code with only awaits in the key.

tests/language/async_switch_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:async";
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 
 foo1(a) async {
   int k = 0;
@@ skipping 32 matching lines @@
       k += 1;
       break;
     case 2:
       k += a;
       return k+2;
     default: k = 2; /// withDefault: ok
   }
   return k;
 }
 
+foo4(value) async {
+  int k = 0;
+  switch(await value) {
+    case 1:
+      k += 1;
+      break;
+    case 2:
+      k += 2;
+      return 2 + k;
+    default: k = 2; /// withDefault: ok
+  }
+  return k;
+}
+
 futureOf(a) async => await a;
 
 test() async {
   Expect.equals(1, await foo1(1));
   Expect.equals(4, await foo1(2));
   Expect.equals(2, await foo1(3)); /// withDefault: ok
   Expect.equals(0, await foo1(3)); /// none: ok
   Expect.equals(1, await foo2(futureOf(1)));
   Expect.equals(4, await foo2(futureOf(2)));
   Expect.equals(2, await foo2(futureOf(3))); /// withDefault: ok
   Expect.equals(0, await foo2(futureOf(3))); /// none: ok
   Expect.equals(1, await foo3(1));
   Expect.equals(4, await foo3(2));
   Expect.equals(2, await foo3(3)); /// withDefault: ok
   Expect.equals(0, await foo3(3)); /// none: ok
+  Expect.equals(1, await foo4(futureOf(1)));
+  Expect.equals(4, await foo4(futureOf(2)));
+  Expect.equals(2, await foo4(futureOf(3))); /// withDefault: ok
+  Expect.equals(0, await foo4(futureOf(3))); /// none: ok
 }
 
 void main() {
   asyncStart();
   test().then((_) => asyncEnd());
 }