Fix an issue with duplicate reported analysis errors.

pkg/analyzer_cli/test/errors_reported_once_test.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:io';
 
 import 'package:analyzer_cli/src/driver.dart';
 import 'package:analyzer_cli/src/options.dart';
 import 'package:path/path.dart' as path;
 import 'package:test/test.dart';
@@ skipping 21 matching lines @@
     errorSink = new StringBuffer();
   }
 
   void tearDown() {
     outSink = savedOutSink;
     errorSink = savedErrorSink;
     exitCode = savedExitCode;
     exitHandler = savedExitHandler;
   }
 
-  @failingTest // #25697
   test_once() async {
     String testDir = path.join(testDirectory, 'data', 'errors_reported_once');
     Driver driver = new Driver();
     await driver.start(
         [path.join(testDir, 'foo.dart'), path.join(testDir, 'bar.dart')]);
 
     expect(exitCode, 0);
 
     // Ensure that we only have one copy of the error.
     final String unusedWarning = 'Unused import';
     String output = outSink.toString();
     expect(output, contains(unusedWarning));
     expect(unusedWarning.allMatches(output).toList(), hasLength(1));
   }
 }