Some improvements to the command-line analyzer's output.

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';
+import 'package:test_reflective_loader/test_reflective_loader.dart';
+
+import 'utils.dart';
+
+main() {
+  defineReflectiveTests(ErrorsReportedOnceTest);
+}
+
+@reflectiveTest
+class ErrorsReportedOnceTest {
+  StringSink savedOutSink, savedErrorSink;
+  int savedExitCode;
+  ExitHandler savedExitHandler;
+
+  void setUp() {
+    savedOutSink = outSink;
+    savedErrorSink = errorSink;
+    savedExitHandler = exitHandler;
+    savedExitCode = exitCode;
+    exitHandler = (code) => exitCode = code;
+    outSink = new StringBuffer();
+    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));
+  }
+}