Avoid using `extension` as an import prefix. (#302)

It is a built-in-identifier with the new static extension methods.
diff --git a/protoc_plugin/test/all_tests.dart b/protoc_plugin/test/all_tests.dart
index 347da54..70f6cc1 100755
--- a/protoc_plugin/test/all_tests.dart
+++ b/protoc_plugin/test/all_tests.dart
@@ -10,7 +10,7 @@
 import 'client_generator_test.dart' as client_generator;
 import 'const_generator_test.dart' as const_generator;
 import 'enum_generator_test.dart' as enum_generator;
-import 'extension_test.dart' as extension;
+import 'extension_test.dart' as extension_test;
 import 'file_generator_test.dart' as file_generator;
 import 'generated_message_test.dart' as generated_message;
 import 'hash_code_test.dart' as hash_code;
@@ -41,7 +41,7 @@
   client_generator.main();
   const_generator.main();
   enum_generator.main();
-  extension.main();
+  extension_test.main();
   file_generator.main();
   generated_message.main();
   hash_code.main();