Improvements to the kernel tree shaker.

tests/language_strong/unused_overridden_async_test.dart

+// Copyright (c) 2016, 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';
+
+class Base {
+  Future<int> method() async {
+    throw 'Should be unused';
+  }
+}
+
+class Sub1 extends Base {
+  @override
+  Future<int> method() async {
+    return 1;
+  }
+}
+
+class Sub2 extends Base {
+  @override
+  Future<int> method() async {
+    return 2;
+  }
+}
+
+help(Base object) async {
+  print(await object.method());
+}
+
+main() async {
+  await help(new Sub1());
+  await help(new Sub2());
+}