Remove code supporting incremental compilation in dart2js

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, STATIC_FUNCTION_NAME_TO_CLOSURE, TYPES;
 
 import '../../common.dart';
@@ skipping 183 matching lines @@
         return null;
     }
   }
 
   @override
   int generatedSize(OutputUnit unit) {
     Fragment key = _emitter.outputBuffers.keys
         .firstWhere((Fragment fragment) => fragment.outputUnit == unit);
     return _emitter.outputBuffers[key].length;
   }
-
-  @override
-  void invalidateCaches() {}
 }