feat(change_detection): json pipe

Closes #1957

modules/angular2/src/facade/lang.dart

@@ -209,7 +209,10 @@ bool assertionsEnabled() {
 // Can't be all uppercase as our transpiler would think it is a special directive...
 class Json {
   static parse(String s) => convert.JSON.decode(s);
-  static String stringify(data) => convert.JSON.encode(data);
+  static String stringify(data) {
+    var encoder = new convert.JsonEncoder.withIndent("  ");
+    return encoder.convert(data);
+  }
 }
 
 class DateWrapper {