Upgrade unittest to be able to use isNotEmpty.

pkg/analysis_server/test/socket_server_test.dart

Index: pkg/analysis_server/test/socket_server_test.dart
diff --git a/pkg/analysis_server/test/socket_server_test.dart b/pkg/analysis_server/test/socket_server_test.dart
index 10b684d3b0a4449ad3b17fdf4d34d15a8ad9926a..047a1c8a59d6d39f363db089391429b030f10c64 100644
--- a/pkg/analysis_server/test/socket_server_test.dart
+++ b/pkg/analysis_server/test/socket_server_test.dart
@@ -95,7 +95,7 @@ class SocketServerTest {
       expect(response.error.code, equals(RequestErrorCode.SERVER_ERROR));
       expect(response.error.message, equals('mock request exception'));
       expect(response.error.stackTrace, isNotNull);
-      expect(response.error.stackTrace, isNot(isEmpty));
+      expect(response.error.stackTrace, isNotEmpty);
       channel.expectMsgCount(responseCount: 1, notificationCount: 1);
     });
   }