Fix all strong mode warnings in protoc-plugin

test/service_test.dart

 library service_test;
 
 import 'dart:async' show Future;
 
 import 'package:protobuf/protobuf.dart';
 import 'package:test/test.dart';
 
 import '../out/protos/service.pb.dart' as pb;
 import '../out/protos/service2.pb.dart' as pb2;
 import '../out/protos/service3.pb.dart' as pb3;
@@ skipping 91 matching lines @@
     var map = service.$messageJson;
     expect(map.keys, [
       '.SearchRequest',
       '.SearchResponse',
       '.service2.SearchRequest',
       '.service2.SearchResponse',
       '.service3.SearchResult',
     ]);
 
     String readMessageName(fqname) {
-      var descriptor = new DescriptorProto()
-        ..mergeFromJsonMap(map[fqname]);
+      var json = map[fqname] as Map<String, dynamic>;
+      var descriptor = new DescriptorProto()..mergeFromJsonMap(json);
       return descriptor.name;
     }
     expect(readMessageName('.SearchRequest'), "SearchRequest");
     expect(readMessageName('.service2.SearchRequest'), "SearchRequest");
     expect(readMessageName('.service3.SearchResult'), "SearchResult");
   });
 }