Add fetch recipe for the Flutter Engine repository.

Tested with the following

$ mkdir flutter_engine
$ cd flutter_engine
$ fetch flutter
$ cd src

 # Confirm the source tree and dependencies are as expected.

BUG=None
TEST=see above

Change-Id: Ie1a6653c3b76354e282688e0db50acc112794426
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3026128Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Chinmay Garde <chinmaygarde@google.com>

fetch_configs/flutter.py

+# Copyright 2021 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 sys
+
+import config_util  # pylint: disable=import-error
+
+
+# This class doesn't need an __init__ method, so we disable the warning
+# pylint: disable=no-init
+class Flutter(config_util.Config):
+  """Basic Config class for the Flutter repository."""
+
+  @staticmethod
+  def fetch_spec(_props):
+    solution = {
+      'custom_deps': {},
+      'deps_file': 'DEPS',
+      'managed'  : False,
+      'name'     : 'src/flutter',
+      'safesync_url': '',
+      'url'      : 'https://github.com/flutter/engine.git',
+    }
+    spec = {
+      'solutions': [solution]
+    }
+    return {
+        'type': 'gclient_git',
+        'gclient_git_spec': spec,
+    }
+
+  @staticmethod
+  def expected_root(_props):
+    return 'src'
+
+
+def main(argv=None):
+  return Flutter().handle_args(argv)
+
+
+if __name__ == '__main__':
+  sys.exit(main(sys.argv))