Use the pure Dart implementation of JSON in leg.

frog/leg/frog_leg.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('frog_leg');
 
 #import('../../lib/uri/uri.dart');
 #import('../lang.dart', prefix: 'frog');
 #import('elements/elements.dart');
 #import('io/io.dart', prefix: 'io');
@@ skipping 83 matching lines @@
 
   Script readScript(Uri uri, [ScriptTag node]) {
     String uriName = uri.toString();
     // TODO(ahe): Clean this up.
     if (uriName == 'dart:dom') {
       uri = getUriFor(io.join([legDirectory, '..', '..',
           'client', 'dom', 'frog', 'dom_frog.dart']));
     } else if (uriName == 'dart:html') {
       uri = getUriFor(io.join([legDirectory, '..', '..',
           'client', 'html', 'frog', 'html_frog.dart']));
+    } else if (uriName == 'dart:json') {
+      uri = getUriFor(io.join([legDirectory, '..', '..',
+          'lib', 'json', 'json.dart']));
     }
     if (uri.scheme != 'file') cancel('cannot read $uri', node: node);
     String text = "";
     try {
       text = frog.world.files.readAll(uri.path);
     } catch (var exception) {
       cancel("${uri.path}: $exception", node: node);
     }
     world.dartBytesRead += text.length;
     frog.SourceFile sourceFile = new frog.SourceFile(uri.toString(), text);
@@ skipping 42 matching lines @@
     LibraryElement library = scanner.loadLibrary(uri, null);
     return library;
   }
 }
 
 class AbortLeg {
   final message;
   AbortLeg(this.message);
   toString() => 'Aborted due to --throw-on-error: $message';
 }