Add a snapshot of flutter/engine/src/build to our sdk

build/PRESUBMIT.py

+# Copyright 2015 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.
+
+
+def _RunTests(input_api, output_api):
+  return (input_api.canned_checks.RunUnitTestsInDirectory(
+          input_api, output_api, '.', whitelist=[r'.+_test.py$']))
+
+
+def CheckChangeOnUpload(input_api, output_api):
+  return _RunTests(input_api, output_api)
+
+
+def CheckChangeOnCommit(input_api, output_api):
+  return _RunTests(input_api, output_api)