Make PbMapMixin implement Map<String, dynamic>

lib/src/protobuf/mixins/map_mixin.dart

Index: lib/src/protobuf/mixins/map_mixin.dart
diff --git a/lib/src/protobuf/mixins/map_mixin.dart b/lib/src/protobuf/mixins/map_mixin.dart
index 0032552f3796c0245f2444230a0bee07a9b9762d..0928681b87bf43dab953bf1e29c5b5b1b6aacd2b 100644
--- a/lib/src/protobuf/mixins/map_mixin.dart
+++ b/lib/src/protobuf/mixins/map_mixin.dart
@@ -11,7 +11,7 @@ import "package:protobuf/protobuf.dart" show BuilderInfo;
 ///
 /// 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_;
@@ -29,8 +29,8 @@ abstract class PbMapMixin implements Map {
   }
 
   @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}");
@@ -39,13 +39,13 @@ abstract class PbMapMixin implements Map {
   }
 
   @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) {