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

dart/frog/leg/api.dart

Index: dart/frog/leg/api.dart
diff --git a/dart/frog/leg/api.dart b/dart/frog/leg/api.dart
index 26185a020a79a5a6613d1728afb7c7c23de7009f..93a792cfeb41b646052af6097770536b85b4fc22 100644
--- a/dart/frog/leg/api.dart
+++ b/dart/frog/leg/api.dart
@@ -39,8 +39,9 @@ typedef void DiagnosticHandler(Uri uri, int begin, int end,
 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>();