Fix a compiler warning.

Change-Id: I4c4e2a4a248e4336fef735f75ff5648d8d33627e

src/google/protobuf/compiler/js/js_generator.cc

@@ -690,6 +690,8 @@ string JSFieldDefault(const FieldDescriptor* field) {
     case FieldDescriptor::CPPTYPE_MESSAGE:
       return "null";
   }
+  GOOGLE_LOG(FATAL) << "Shouldn't reach here.";
+  return "";
 }
 
 string ProtoTypeName(const GeneratorOptions& options,