`flutter analyze` cleanup (#20490)

* `flutter analyze` cleanup

* Make `--dartdocs` work in all modes.
* Make `analyze-sample-code.dart` more resilient.
* Add a test for `analyze-sample-code.dart`.
* Minor cleanup in related code and files.

* Apply review comments

* Fix tests

dev/bots/test/analyze-sample-code_test.dart

diff --git a/dev/bots/test/analyze-sample-code_test.dart b/dev/bots/test/analyze-sample-code_test.dart
new file mode 100644
index 0000000..0774eea
--- /dev/null
+++ b/dev/bots/test/analyze-sample-code_test.dart
@@ -0,0 +1,67 @@
+// Copyright 2018 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:convert';
+import 'dart:io';
+
+import 'common.dart';
+
+void main() {
+  test('analyze-sample-code', () async {
+    final Process process = await Process.start(
+      '../../bin/cache/dart-sdk/bin/dart',
+      <String>['analyze-sample-code.dart', 'test/analyze-sample-code-test-input'],
+    );
+    final List<String> stdout = await process.stdout.transform(utf8.decoder).transform(const LineSplitter()).toList();
+    final List<String> stderr = await process.stderr.transform(utf8.decoder).transform(const LineSplitter()).toList();
+    final Match line = new RegExp(r'^(.+)/main\.dart:[0-9]+:[0-9]+: .+$').matchAsPrefix(stdout[1]);
+    expect(line, isNot(isNull));
+    final String directory = line.group(1);
+    new Directory(directory).deleteSync(recursive: true);
+    expect(await process.exitCode, 1);
+    expect(stderr, isEmpty);
+    expect(stdout, <String>[
+      'Found 2 sample code sections.',
+      "$directory/main.dart:1:8: Unused import: 'dart:async'",
+      "$directory/main.dart:2:8: Unused import: 'dart:convert'",
+      "$directory/main.dart:3:8: Unused import: 'dart:math'",
+      "$directory/main.dart:4:8: Unused import: 'dart:typed_data'",
+      "$directory/main.dart:5:8: Unused import: 'dart:ui'",
+      "$directory/main.dart:6:8: Unused import: 'package:flutter_test/flutter_test.dart'",
+      "$directory/main.dart:9:8: Target of URI doesn't exist: 'package:flutter/known_broken_documentation.dart'",
+      "test/analyze-sample-code-test-input/known_broken_documentation.dart:27:9: Undefined class 'Opacity' (undefined_class)",
+      "test/analyze-sample-code-test-input/known_broken_documentation.dart:29:20: Undefined class 'Text' (undefined_class)",
+      "test/analyze-sample-code-test-input/known_broken_documentation.dart:39:9: Undefined class 'Opacity' (undefined_class)",
+      "test/analyze-sample-code-test-input/known_broken_documentation.dart:41:20: Undefined class 'Text' (undefined_class)",
+      'test/analyze-sample-code-test-input/known_broken_documentation.dart:42:5: unexpected comma at end of sample code',
+      'Kept $directory because it had errors (see above).',
+      '-------8<-------',
+      '     1: // generated code',
+      "     2: import 'dart:async';",
+      "     3: import 'dart:convert';",
+      "     4: import 'dart:math' as math;",
+      "     5: import 'dart:typed_data';",
+      "     6: import 'dart:ui' as ui;",
+      "     7: import 'package:flutter_test/flutter_test.dart';",
+      '     8: ',
+      '     9: // test/analyze-sample-code-test-input/known_broken_documentation.dart',
+      "    10: import 'package:flutter/known_broken_documentation.dart';",
+      '    11: ',
+      '    12: bool _visible = true;',
+      '    13: dynamic expression1 = ',
+      '    14: new Opacity(',
+      '    15:   opacity: _visible ? 1.0 : 0.0,',
+      "    16:   child: const Text('Poor wandering ones!'),",
+      '    17: )',
+      '    18: ;',
+      '    19: dynamic expression2 = ',
+      '    20: new Opacity(',
+      '    21:   opacity: _visible ? 1.0 : 0.0,',
+      "    22:   child: const Text('Poor wandering ones!'),",
+      '    23: ),',
+      '    24: ;',
+      '-------8<-------',
+    ]);
+  }, skip: !Platform.isLinux);
+}