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

build/config/ios/ios_sdk.py

+# Copyright 2014 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 subprocess
+import sys
+
+# This script returns the path to the SDK of the given type. Pass the type of
+# SDK you want, which is typically "iphone" or "iphonesimulator".
+#
+# In the GYP build, this is done inside GYP itself based on the SDKROOT
+# variable.
+
+if len(sys.argv) != 2:
+  print "Takes one arg (SDK to find)"
+  sys.exit(1)
+
+print subprocess.check_output(['xcodebuild', '-version', '-sdk',
+                               sys.argv[1], 'Path']).strip()