Add "mock" compilation feature to dart2js and use it to improve test coverage.

dart/frog/leg/api.dart

 // Copyright (c) 2012, 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.
 
 #library('leg_api');
 
 #import('../../lib/uri/uri.dart');
 #import('apiimpl.dart');
 
 // Unless explicitly allowed, passing null for any argument to the
@@ skipping 21 matching lines @@
                                String message, bool fatal);
 
 /**
  * Returns [script] compiled to JavaScript. If the compilation fails,
  * null is returned and [handler] will have been invoked at least once
  * with [:fatal == true:].
  */
 Future<String> compile(Uri script,
                        Uri libraryRoot,
                        ReadUriFromString provider,
-                       DiagnosticHandler handler) {
-  Compiler compiler = new Compiler(provider, handler, libraryRoot);
+                       DiagnosticHandler handler,
+                       [List<String> options = const []]) {
+  Compiler compiler = new Compiler(provider, handler, libraryRoot, options);
   compiler.run(script);
   String code = compiler.assembledCode;
   Completer<String> completer = new Completer<String>();
   completer.complete(code);
   return completer.future;
 }