Fix failing test/completion_test.dart

Change-Id: I12676b8d1422d9d521f8c1ac84a28b30237b1ee6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/196242
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov

pkg/analysis_server/test/completion_test.dart

@@ -2443,9 +2443,10 @@ class A<Z extends X> {
         ++expectedFailCount;
         test('$testName (expected failure $expectedFailCount)', () {
           var test = CompletionTestCase();
-          return Future(() => test.runTest(spec, extraFiles)).then((_) {
-            fail('Test passed - expected to fail.');
-          }, onError: (_) {});
+          expect(
+            () => test.runTest(spec, extraFiles),
+            throwsA(anything),
+          );
         });
       } else {
         ++expectedPassCount;