Test framework for analyze.dart (#44772)

* Prepare analyze.dart for testing.

* Reorder the file for easier understanding

* Add a test for analyze.dart

* Fix review comments

dev/bots/test/analyze-test-input/packages/foo/foo.dart

+// THIS FILE HAS NO LICENSE.
+// IT IS PART OF A TEST FOR MISSING LICENSES.

dev/bots/test/analyze_test.dart

+// Copyright 2017 The Chromium Authors. 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:path/path.dart' as path;
+
+import '../analyze.dart';
+import 'common.dart';
+
+typedef AsyncVoidCallback = Future<void> Function();
+
+Future<String> capture(AsyncVoidCallback callback, { int exitCode = 0 }) async {
+  final StringBuffer buffer = StringBuffer();
+  final PrintCallback oldPrint = print;
+  try {
+    print = (Object line) {
+      buffer.writeln(line);
+    };
+    try {
+      await callback();
+      expect(exitCode, 0);
+    } on ExitException catch (error) {
+      expect(error.exitCode, exitCode);
+    }
+  } finally {
+    print = oldPrint;
+  }
+  return buffer.toString();
+}
+
+void main() {
+  test('analyze.dart - verifyNoMissingLicense', () async {
+    final String result = await capture(() => verifyNoMissingLicense(path.join('test', 'analyze-test-input')), exitCode: 1);
+    expect(result, '''
+━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+License headers cannot be found at the beginning of the following file.
+
+test/analyze-test-input/packages/foo/foo.dart
+━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+
+'''.replaceAll('/', Platform.isWindows ? '\\' : '/'));
+  });
+}