Fix problems with default-cases in async code.

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;
+  switch(a) {
+    case 1:
+      await 3;
+      k += 1;
+      break;
+    case 2:
+      k += a;
+      return k+2;
+    default: k = 2; /// withDefault: ok
+  }
+  return k;
+}
+
+foo2(a) async {
+  int k = 0;
+  switch(await a) {
+    case 1:
+      await 3;
+      k += 1;
+      break;
+    case 2:
+      k += await a;
+      return k+2;
+    default: k = 2; /// withDefault: ok
+  }
+  return k;
+}
+
+foo3(a) async {
+  int k = 0;
+  switch(a) {
+    case 1:
+      k += 1;
+      break;
+    case 2:
+      k += a;
+      return k+2;
+    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
+}
+
+void main() {
+  asyncStart();
+  test().then((_) => asyncEnd());
+}