Fix 56 hints in pkg/compiler

pkg/compiler/lib/src/js_backend/codegen/codegen.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 code_generator;
 
 import 'glue.dart';
 
 import '../../tree_ir/tree_ir_nodes.dart' as tree_ir;
 import '../../js/js.dart' as js;
@@ skipping 288 matching lines @@
     return js.js('#.#.call(#, #)',
         [glue.prototypeAccess(node.target.enclosingClass),
          glue.invocationName(node.selector),
          visitExpression(node.receiver),
          visitArguments(node.arguments)]);
   }
 
   @override
   js.Expression visitLiteralList(tree_ir.LiteralList node) {
     registry.registerInstantiatedClass(glue.listClass);
-    int length = node.values.length;
     List<js.Expression> entries = node.values.map(visitExpression).toList();
     return new js.ArrayInitializer(entries);
   }
 
   @override
   js.Expression visitLiteralMap(tree_ir.LiteralMap node) {
     ConstructorElement constructor;
     if (node.entries.isEmpty) {
       constructor = glue.mapLiteralConstructorEmpty;
     } else {
@@ skipping 353 matching lines @@
 
   @override
   visitVariableDeclaration(tree_ir.VariableDeclaration node) {
     return errorUnsupportedNode(node);
   }
 
   errorUnsupportedNode(tree_ir.DartSpecificNode node) {
     throw "Unsupported node in JS backend: $node";
   }
 }