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

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

Index: pkg/compiler/lib/src/ssa/invoke_dynamic_specializers.dart
diff --git a/pkg/compiler/lib/src/ssa/invoke_dynamic_specializers.dart b/pkg/compiler/lib/src/ssa/invoke_dynamic_specializers.dart
index 12f842ad7069f2a87fce3a4042f99e4533e4f52e..fa9ce3d5a41fe3e89beb63abc0b9738e19ffead9 100644
--- a/pkg/compiler/lib/src/ssa/invoke_dynamic_specializers.dart
+++ b/pkg/compiler/lib/src/ssa/invoke_dynamic_specializers.dart
@@ -2,12 +2,10 @@
 // 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';