Mark static function getters as used in codegen.

tests/compiler/dart2js/top_level_closure_tree_shake_test.dart

+// Copyright (c) 2013, 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';
+import 'compiler_helper.dart';
+
+const String TEST_ONE = r"""
+main() {
+  var f = use;
+  if (false) {
+    // This statement and the use of 'foo' should be optimized away, causing
+    // 'foo' to be absent from the final code.
+    f(foo);
+  }
+  f(bar);
+}
+
+foo() => 'Tarantula!';
+bar() => 'Coelacanth!';
+
+use(x) {
+  print(x());
+}
+""";
+
+
+main() {
+  asyncTest(() => Future.wait([
+      compileAll(TEST_ONE).then((String generated) {
+        Expect.isFalse(generated.contains('Tarantula!'),
+            "failed to remove 'foo'");
+        Expect.isTrue(generated.contains('Coelacanth!'));
+      }),
+  ]));
+}