dart2js: Fix hints in code base.

pkg/compiler/lib/src/dart_backend/backend_ast_to_frontend_ast.dart

 // Copyright (c) 2014, 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 dart_tree_printer;
 
 import '../constants/values.dart' as values;
 import '../dart_types.dart' as types;
 import '../dart2jslib.dart' as dart2js;
 import '../elements/elements.dart' as elements;
@@ skipping 1195 matching lines @@
     types.InterfaceType supertype = cls.supertype;
     Set<types.DartType> mixinTypes = new Set<types.DartType>();
     Link<tree.Node> mixins = const Link<tree.Node>();
 
     void addMixin(types.DartType mixinType) {
       mixinTypes.add(mixinType);
       mixins = mixins.prepend(makeType(TypeGenerator.createType(mixinType)));
     }
 
     if (supertype != null) {
-      tree.Node superclass;
       if (supertype.element.isUnnamedMixinApplication) {
         while (supertype.element.isUnnamedMixinApplication) {
           elements.MixinApplicationElement mixinApplication = supertype.element;
           addMixin(cls.asInstanceOf(mixinApplication.mixin));
           supertype = mixinApplication.supertype;
         }
         tree.Node superclass = makeType(
             TypeGenerator.createType(cls.asInstanceOf(supertype.element)));
         supernode = new tree.MixinApplication(
             superclass, new tree.NodeList(null, mixins, null, ','));
@@ skipping 105 matching lines @@
             printStringChunk(chunk.previous),
             node);
       } else {
         return node;
       }
     }
     return printStringChunk(output.chunk);
   }
 
 }