Add a way to opt a file out of Dart formatting

Author: Hixie

script/tool/CHANGELOG.md

@@ -10,6 +10,8 @@
   `--no-push-flags`. Releases now always tag and push.
 - **Breaking change**: `publish`'s `--package` flag has been replaced with the
   `--packages` flag used by most other packages.
+- Formatting now skips files that contain a line that exactly
+  matches the string `// This file is hand-formatted`.
 
 ## 0.5.0
 

script/tool/lib/src/format_command.dart

@@ -206,7 +206,21 @@ class FormatCommand extends PluginCommand {
 
     final String fromPath = relativeTo.path;
 
+    const String handFormattedPragma = '// This file is hand-formatted.';
+
     return files
+        .where((File file) {
+          try {
+            return !file.readAsLinesSync().contains(handFormattedPragma);
+          } catch FileSystemException {
+            // If we can't open the file or can't read it as UTF-8, then we just
+            // consider it included. It should get filtered out later when we
+            // deal with file extensions. (We want the formatters to see the
+            // files that we can't read, in case they have opinions about such
+            // things as character encodings, etc.)
+            return true;
+          }
+        })
         .map((File file) => path.relative(file.path, from: fromPath))
         .where((String path) =>
             // Ignore files in build/ directories (e.g., headers of frameworks)

script/tool/test/format_command_test.dart

@@ -8,6 +8,7 @@ import 'package:args/command_runner.dart';
 import 'package:file/file.dart';
 import 'package:file/memory.dart';
 import 'package:flutter_plugin_tools/src/common/core.dart';
+import 'package:flutter_plugin_tools/src/common/file_utils.dart';
 import 'package:flutter_plugin_tools/src/format_command.dart';
 import 'package:path/path.dart' as p;
 import 'package:test/test.dart';
@@ -85,12 +86,21 @@ void main() {
       'lib/src/b.dart',
       'lib/src/c.dart',
     ];
+    const String fileWithOptOut = 'lib/src/d.dart';
     final Directory pluginDir = createFakePlugin(
       'a_plugin',
       packagesDir,
-      extraFiles: files,
+      extraFiles: <String>[
+        ...files,
+        fileWithOptOut,
+      ],
     );
 
+    final p.Context posixContext = p.posix;
+    childFileWithSubcomponents(pluginDir, posixContext.split(fileWithOptOut))
+        .writeAsStringSync(
+            '// copyright bla bla\n// This file is hand-formatted.\ncode...');
+
     await runCapturingPrint(runner, <String>['format']);
 
     expect(