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

dart/frog/leg/frog_leg.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('frog_leg');
 
 #import('../../lib/uri/uri.dart');
 #import('../lang.dart', prefix: 'frog');
 #import('api.dart', prefix: 'api');
 #import('io/io.dart', prefix: 'io');
@@ skipping 24 matching lines @@
     }
     sb.add('${uriParts.last()}');
     return sb.toString();
   }
   return uri.toString();
 }
 
 bool compile(frog.World world) {
   final throwOnError = frog.options.throwOnErrors;
   final showWarnings = frog.options.showWarnings;
+  final allowMockCompilation = frog.options.allowMockCompilation;
   // final compiler = new WorldCompiler(world, throwOnError);
   Uri cwd = new Uri(scheme: 'file', path: io.getCurrentDirectory());
   Uri uri = cwd.resolve(frog.options.dartScript);
   String frogLibDir = frog.options.libDir;
   if (!frogLibDir.endsWith("/")) frogLibDir = "$frogLibDir/";
   Uri frogLib = new Uri(scheme: 'file', path: frogLibDir);
   Uri libraryRoot = frogLib.resolve('../leg/lib/');
   Map<String, frog.SourceFile> sourceFiles = <frog.SourceFile>{};
 
   Future<String> provider(Uri uri) {
@@ skipping 17 matching lines @@
     if (uri === null) {
       assert(fatal);
       print(message);
     } else if (fatal || showWarnings) {
       frog.SourceFile file = sourceFiles[uri.toString()];
       print(file.getLocationMessage(message, begin, end, true));
     }
     if (fatal && throwOnError) throw new AbortLeg(message);
   }
 
+  List<String> options = new List<String>();
+  if (allowMockCompilation) options.add('--allow-mock-compilation');
+
   // TODO(ahe): We expect the future to be complete and call value
   // directly. In effect, we don't support truly asynchronous API.
-  String code = api.compile(uri, libraryRoot, provider, handler).value;
+  String code = api.compile(uri, libraryRoot, provider, handler, options).value;
   if (code === null) return false;
   world.legCode = code;
   world.jsBytesWritten = code.length;
   return true;
 }
 
 class AbortLeg {
   final message;
   AbortLeg(this.message);
   toString() => 'Aborted due to --throw-on-error: $message';
 }