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

pkg/compiler/lib/src/js_backend/interceptor_data.dart

 // Copyright (c) 2017, 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 js_backend.interceptor_data;
 
 import '../common/names.dart' show Identifiers;
-import '../common/resolution.dart' show Resolution;
 import '../common_elements.dart' show CommonElements, ElementEnvironment;
 import '../elements/elements.dart' show MemberElement;
 import '../elements/entities.dart';
 import '../elements/types.dart';
 import '../js/js.dart' as jsAst;
 import '../types/types.dart' show TypeMask;
 import '../universe/selector.dart';
 import '../world.dart' show ClosedWorld;
 import 'backend_helpers.dart';
 import 'namer.dart';
@@ skipping 309 matching lines @@
     jsAst.Name name = namer.nameForGetInterceptor(classes);
     if (classes.contains(_helpers.jsInterceptorClass)) {
       // We can't use a specialized [getInterceptorMethod], so we make
       // sure we emit the one with all checks.
       _specializedGetInterceptors[name] = _interceptorData.interceptedClasses;
     } else {
       _specializedGetInterceptors[name] = classes;
     }
   }
 }