dart2js: Fix hints in code base.

pkg/compiler/lib/src/closure.dart

 // Copyright (c) 2012, 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 closureToClassMapper;
 
 import 'constants/expressions.dart';
 import 'dart2jslib.dart';
 import 'dart_types.dart';
 import 'elements/elements.dart';
@@ skipping 70 matching lines @@
     } else {
       throw new SpannableAssertionFailure(
           closure, 'Not a closure: $closure (${closure.runtimeType}).');
     }
     compiler.enqueuer.codegen.forgetElement(cls);
   }
 }
 
 /// Common interface for [BoxFieldElement] and [ClosureFieldElement] as
 /// non-elements.
-abstract class CapturedVariable {}
+abstract class CapturedVariable implements Element {}
 
 // TODO(ahe): These classes continuously cause problems.  We need to
 // find a more general solution.
 class ClosureFieldElement extends ElementX
     implements FieldElement, CapturedVariable {
   /// The [BoxLocal] or [LocalElement] being accessed through the field.
   final Local local;
 
   ClosureFieldElement(String name,
                       this.local,
@@ skipping 1009 matching lines @@
 
   String get name => typeVariable.name;
 
   int get hashCode => typeVariable.hashCode;
 
   bool operator ==(other) {
     if (other is! TypeVariableLocal) return false;
     return typeVariable == other.typeVariable;
   }
 }