Fix all strong mode warnings in protoc-plugin

test/service_test.dart

Index: test/service_test.dart
diff --git a/test/service_test.dart b/test/service_test.dart
index 969bb5be7678c0fa63aeb58610523b86adea870d..2628a1f054052bf0e56d3dc8ed02d2a8e22078d9 100644
--- a/test/service_test.dart
+++ b/test/service_test.dart
@@ -109,8 +109,8 @@ void main() {
     ]);
 
     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");