Make json-maps implement Map<String, dynamic>.

runtime/lib/convert_patch.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.
 
 import "dart:_internal" show POWERS_OF_TEN;
 
 // JSON conversion.
 
 @patch
 _parseJson(String source, reviver(key, value)) {
@@ skipping 124 matching lines @@
   void handleBool(bool value) {
     this.value = value;
   }
 
   void handleNull() {
     this.value = null;
   }
 
   void beginObject() {
     pushContainer();
-    currentContainer = {};
+    currentContainer = <String, dynamic>{};
   }
 
   void propertyName() {
     key = value;
     value = null;
   }
 
   void propertyValue() {
     Map map = currentContainer;
     map[key] = value;
@@ skipping 1645 matching lines @@
     _parser.parse(start);
   }
 
   void close() {
     _parser.close();
     var decoded = _parser.result;
     _sink.add(decoded);
     _sink.close();
   }
 }