fix webui for sdk 0.8.7

lib/src/html_css_fixup.dart

 // Copyright (c) 2013, 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 html_css_fixup;
 
-import 'dart:json' as json;
+import 'dart:convert';
 
 import 'package:csslib/parser.dart' as css;
 import 'package:csslib/visitor.dart';
 import 'package:html5lib/dom.dart';
 import 'package:html5lib/dom_parsing.dart';
 
 import 'compiler.dart';
 import 'emitters.dart';
 import 'info.dart';
 import 'messages.dart';
@@ skipping 79 matching lines @@
  */
 String createCssSelectorsExpression(ComponentInfo info, bool mangled) {
   var cssVisited = new IdClassVisitor();
 
   // For components only 1 stylesheet allowed.
   if (!info.styleSheets.isEmpty && info.styleSheets.length == 1) {
     var styleSheet = info.styleSheets[0];
     cssVisited..visitTree(styleSheet);
   }
 
-  return json.stringify(_createCssSimpleSelectors(cssVisited, info, mangled));
+  return JSON.encode(_createCssSimpleSelectors(cssVisited, info, mangled));
 }
 
 // TODO(terry): Need to handle other selectors than IDs/classes like tag name
 //              e.g., DIV { color: red; }
 // TODO(terry): Would be nice if we didn't need to mangle names; requires users
 //              to be careful in their code and makes it more than a "polyfill".
 //              Maybe mechanism that generates CSS class name for scoping.  This
 //              would solve tag name selectors (see above TODO).
 /**
  * Fix a component's HTML to implement scoped stylesheets.
@@ skipping 448 matching lines @@
 
         // Find all imports return list of @imports in this style tag.
         var urlInfos = findImportsInStyleSheet(styleSheet, _packageRoot,
             _inputUrl, _messages);
         imports.addAll(urlInfos);
       }
     }
     super.visitElement(node);
   }
 }