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

pkg/compiler/lib/src/js_backend/no_such_method_registry.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.
 
 import '../common.dart';
 import '../common/names.dart' show Identifiers, Names, Selectors;
-import '../common_elements.dart';
 import '../elements/elements.dart';
 import '../elements/entities.dart';
 import '../types/types.dart';
 import '../tree/tree.dart';
 import 'backend_helpers.dart';
 
 /**
  * Categorizes `noSuchMethod` implementations.
  *
  * If user code includes `noSuchMethod` implementations, type inference is
@@ skipping 67 matching lines @@
   void onQueueEmpty() {
     _uncategorizedImpls.forEach(_categorizeImpl);
     _uncategorizedImpls.clear();
   }
 
   /// Now that type inference is complete, split category D into two
   /// subcategories: D1, those that have no return type, and D2, those
   /// that have a return type.
   void onTypeInferenceComplete(GlobalTypeInferenceResults results) {
     otherImpls.forEach((FunctionEntity element) {
-      if (results.resultOfMember(element as MemberEntity).throwsAlways) {
+      if (results.resultOfMember(
+          // ignore: UNNECESSARY_CAST
+          element as MemberEntity).throwsAlways) {
         complexNoReturnImpls.add(element);
       } else {
         complexReturningImpls.add(element);
       }
     });
   }
 
   /// Emits a diagnostic
   void emitDiagnostic(DiagnosticReporter reporter) {
     throwingImpls.forEach((e) {
@@ skipping 174 matching lines @@
         return stmt.expression is Throw;
       }
     }
     return false;
   }
 
   MethodElement getSuperNoSuchMethod(MethodElement method) {
     return method.enclosingClass.lookupSuperByName(Names.noSuchMethod_);
   }
 }