Add ability to pass flags to et run (flutter#185109)

Adds `--flutter-flags` flag to `et run` that allows you to pass command
line flags to the `flutter run` call it makes under the hood. For
example, `et run --flutter-flags="--trace-to-file=trace.txt
--verbose-system-logs"`.

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we expect every widget to implement].
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- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
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Fixes] where supported.
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---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: camsim99

engine/src/flutter/tools/engine_tool/lib/src/commands/run_command.dart

@@ -29,6 +29,15 @@ final class RunCommand extends CommandBase {
     @visibleForTesting FlutterTool? flutterTool,
   }) {
     builds = BuildPlan.configureArgParser(argParser, environment, configs: configs, help: help);
+    argParser.addMultiOption(
+      'flutter-flags',
+      help:
+          'Arguments to pass to the "flutter run" command. '
+          'This can be specified multiple times. For multiple flags, '
+          'separate them by spaces, e.g. '
+          '--flutter-flags="--profile --verbose".',
+      splitCommas: false,
+    );
     _flutterTool = flutterTool ?? FlutterTool.fromEnvironment(environment);
   }
 
@@ -169,9 +178,16 @@ See `flutter run --help` for a listing
       mangledBuildName,
       '--local-engine-host',
       mangledHostBuildName,
-      ...argResults!.rest,
     ];
 
+    // Add flags passed to --flutter-flags.
+    final flutterFlags = argResults!['flutter-flags'] as List<String>;
+    for (final flagGroup in flutterFlags) {
+      command.addAll(flagGroup.split(' ').where((String s) => s.isNotEmpty));
+    }
+
+    command.addAll(argResults!.rest);
+
     // TODO(johnmccutchan): Be smart and if the user requested a profile
     // config, add the '--profile' flag when invoking flutter run.
     final ProcessRunnerResult result = await environment.processRunner.runProcess(

engine/src/flutter/tools/engine_tool/test/commands/run_command_test.dart

@@ -369,6 +369,23 @@ void main() {
       );
     });
 
+    test('delegates to `flutter run` with --flutter-flags', () async {
+      await et.run([
+        'run',
+        '--config=android_debug_arm64',
+        '--flutter-flags=--foo',
+        '--flutter-flags=--bar=baz,foobar',
+        '--flutter-flags=--flag1 --flag2',
+      ]);
+
+      expect(
+        commandsRun,
+        containsAllInOrder([
+          containsAllInOrder(['flutter', 'run', '--foo', '--bar=baz,foobar', '--flag1', '--flag2']),
+        ]),
+      );
+    });
+
     group('delegates to `flutter run` in mode', () {
       for (final mode in const ['debug', 'profile', 'release']) {
         test('$mode mode', () async {