Make PbMapMixin implement Map<String, dynamic>

lib/src/protobuf/mixins/map_mixin.dart

 // Copyright (c) 2015, 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 protobuf.mixins.map;
 
 import "package:protobuf/protobuf.dart" show BuilderInfo;
 
 /// A PbMapMixin provides an experimental implementation of the
 /// Map interface for a GeneratedMessage.
 ///
 /// This mixin is enabled via an option in
 /// dart_options.proto in dart-protoc-plugin.
-abstract class PbMapMixin implements Map {
+abstract class PbMapMixin implements Map<String, dynamic> {
   // GeneratedMessage properties and methods used by this mixin.
 
   BuilderInfo get info_;
   void clear();
   int getTagNumber(String fieldName);
   getField(int tagNumber);
   void setField(int tagNumber, var value);
 
   @override
   operator [](key) {
     if (key is! String) return null;
     var tag = getTagNumber(key);
     if (tag == null) return null;
     return getField(tag);
   }
 
   @override
-  operator []=(key, val) {
-    var tag = getTagNumber(key as String);
+  void operator []=(String key, val) {
+    var tag = getTagNumber(key);
     if (tag == null) {
       throw new ArgumentError(
           "field '${key}' not found in ${info_.messageName}");
     }
     setField(tag, val);
   }
 
   @override
-  get keys => info_.byName.keys;
+  Iterable<String> get keys => info_.byName.keys;
 
   @override
   bool containsKey(Object key) => info_.byName.containsKey(key);
 
   @override
-  get length => info_.byName.length;
+  int get length => info_.byName.length;
 
   @override
   remove(key) {
     throw new UnsupportedError(
         "remove() not supported by ${info_.messageName}");
   }
 }