dart2js: Add test to compile empty main without any libraries.

tests/compiler/dart2js/compile_with_empty_libraries_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.
+
+/// Test that the dart2js compiler can compile an empty script without
+/// references to core library definitions.
+
+import 'package:async_helper/async_helper.dart';
+import 'mock_compiler.dart';
+
+const String TEST = r"main() {}";
+
+main() {
+  Uri uri = new Uri(scheme: 'source');
+  MockCompiler compiler =
+    new MockCompiler.internal(librariesOverride: (_) => '');
+  asyncTest(() => compiler.runCompiler(uri));
+}