Fix dart2js warnings and add test to ensure it stays clean.

pkg/compiler/lib/src/ssa/invoke_dynamic_specializers.dart

 // Copyright (c) 2013, 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.
 
-import '../compiler.dart' show Compiler;
 import '../constants/constant_system.dart';
 import '../constants/values.dart';
 import '../elements/elements.dart' show Name;
 import '../elements/entities.dart';
-import '../js_backend/js_backend.dart';
 import '../js_backend/backend_helpers.dart';
 import '../options.dart';
 import '../types/types.dart';
 import '../universe/call_structure.dart';
 import '../universe/selector.dart';
 import '../world.dart' show ClosedWorld;
 import 'nodes.dart';
 import 'types.dart';
 
 /**
@@ skipping 1113 matching lines @@
       ClosedWorld closedWorld) {
     HInstruction receiver = instruction.getDartReceiver(closedWorld);
     if (receiver.isNumberOrNull(closedWorld)) {
       // Even if there is no builtin equivalent instruction, we know the
       // instruction does not have any side effect, and that it can be GVN'ed.
       clearAllSideEffects(instruction);
     }
     return null;
   }
 }