Use the pure Dart implementation of JSON in leg.

frog/leg/frog_leg.dart

Index: frog/leg/frog_leg.dart
===================================================================
--- frog/leg/frog_leg.dart	(revision 5121)
+++ frog/leg/frog_leg.dart	(working copy)
@@ -101,6 +101,9 @@
     } 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 = "";