Add some options to format subcommand (#18360)

* Add `--dry-run` option to `flutter format` sub command

* Add `--set-exit-if-changed` option to `format` sub command

* Add `--machine` option to `format` sub command

* Make variable names to be not shorthand: cmd -> command

Fix https://github.com/flutter/flutter/pull/18360#discussion_r199656120

packages/flutter_tools/lib/src/commands/format.dart

@@ -10,6 +10,26 @@ import '../dart/sdk.dart';
 import '../runner/flutter_command.dart';
 
 class FormatCommand extends FlutterCommand {
+  FormatCommand() {
+    argParser.addFlag('dry-run',
+      abbr: 'n',
+      help: 'Show which files would be modified but make no changes.',
+      defaultsTo: false,
+      negatable: false,
+    );
+    argParser.addFlag('set-exit-if-changed',
+      help: 'Return exit code 1 if there are any formatting changes.',
+      defaultsTo: false,
+      negatable: false,
+    );
+    argParser.addFlag('machine',
+      abbr: 'm',
+      help: 'Produce machine-readable JSON output.',
+      defaultsTo: false,
+      negatable: false,
+    );
+  }
+
   @override
   final String name = 'format';
 
@@ -36,8 +56,25 @@ class FormatCommand extends FlutterCommand {
     }
 
     final String dartfmt = sdkBinaryName('dartfmt');
-    final List<String> cmd = <String>[dartfmt, '-w']..addAll(argResults.rest);
-    final int result = await runCommandAndStreamOutput(cmd);
+    final List<String> command = <String>[dartfmt];
+
+    if (argResults['dry-run']) {
+      command.add('-n');
+    }
+    if (argResults['machine']) {
+      command.add('-m');
+    }
+    if (!argResults['dry-run'] && !argResults['machine']) {
+      command.add('-w');
+    }
+
+    if (argResults['set-exit-if-changed']) {
+      command.add('--set-exit-if-changed');
+    }
+
+    command..addAll(argResults.rest);
+
+    final int result = await runCommandAndStreamOutput(command);
     if (result != 0)
       throwToolExit('Formatting failed: $result', exitCode: result);
   }

packages/flutter_tools/test/commands/format_test.dart

@@ -38,5 +38,42 @@ void main() {
       final String formatted = srcFile.readAsStringSync();
       expect(formatted, original);
     });
+
+    testUsingContext('dry-run', () async {
+      final String projectPath = await createProject(temp);
+
+      final File srcFile = fs.file(
+          fs.path.join(projectPath, 'lib', 'main.dart'));
+      final String nonFormatted = srcFile.readAsStringSync().replaceFirst(
+          'main()', 'main(  )');
+      srcFile.writeAsStringSync(nonFormatted);
+
+      final FormatCommand command = new FormatCommand();
+      final CommandRunner<Null> runner = createTestCommandRunner(command);
+      await runner.run(<String>['format', '--dry-run', srcFile.path]);
+
+      final String shouldNotFormatted = srcFile.readAsStringSync();
+      expect(shouldNotFormatted, nonFormatted);
+    });
+
+    testUsingContext('dry-run with set-exit-if-changed', () async {
+      final String projectPath = await createProject(temp);
+
+      final File srcFile = fs.file(
+          fs.path.join(projectPath, 'lib', 'main.dart'));
+      final String nonFormatted = srcFile.readAsStringSync().replaceFirst(
+          'main()', 'main(  )');
+      srcFile.writeAsStringSync(nonFormatted);
+
+      final FormatCommand command = new FormatCommand();
+      final CommandRunner<Null> runner = createTestCommandRunner(command);
+
+      expect(runner.run(<String>[
+        'format', '--dry-run', '--set-exit-if-changed', srcFile.path
+      ]), throwsException);
+
+      final String shouldNotFormatted = srcFile.readAsStringSync();
+      expect(shouldNotFormatted, nonFormatted);
+    });
   });
 }