dart2js: support tear-offs in the startup emitter.

pkg/compiler/lib/src/js_emitter/startup_emitter/emitter.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.
 
 library dart2js.js_emitter.startup_emitter;
 
 import 'package:js_runtime/shared/embedded_names.dart' show
     JsBuiltin,
     METADATA,
     TYPES;
@@ skipping 24 matching lines @@
   final ModelEmitter _emitter;
 
   JavaScriptBackend get _backend => _compiler.backend;
 
   Emitter(Compiler compiler, Namer namer, NativeEmitter nativeEmitter)
       : this._compiler = compiler,
         this.namer = namer,
         _emitter = new ModelEmitter(compiler, namer, nativeEmitter);
 
   @override
+  String get patchVersion => "startup";
+
+  @override
   int emitProgram(ProgramBuilder programBuilder) {
     Program program = programBuilder.buildProgram();
     return _emitter.emitProgram(program);
   }
 
   @override
   bool get supportsReflection => false;
 
   @override
   bool isConstantInlinedOrAlreadyEmitted(ConstantValue constant) {
@@ skipping 121 matching lines @@
         _compiler.internalError(NO_LOCATION_SPANNABLE,
             "Unhandled Builtin: $builtin");
         return null;
     }
   }
 
   @override
   void invalidateCaches() {
   }
 }