dart2js: Don't emit static const fields since they are inlined where they are used

tests/language/emit_const_fields_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.
+// Test that used static consts are emitted.
+
+class Guide {
+  static const LTUAE = 42;
+  static const TITLE = "Life, the Universe and Everything";
+  static const EARTH = const {
+    "Sector": "ZZ9 Plural Z Alpha",
+    "Status": const [ "Scheduled for demolition", "1978-03-08" ],
+    "Description": "Mostly harmless"
+  };
+}
+
+main() {
+  Expect.isTrue(42 == Guide.LTUAE);
+  Expect.isTrue("1978-03-08" == Guide.EARTH["Status"][1]);
+}
+