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

build/mac/edit_xibs.sh

Index: build/mac/edit_xibs.sh
diff --git a/build/mac/edit_xibs.sh b/build/mac/edit_xibs.sh
new file mode 100755
index 0000000000000000000000000000000000000000..b7b749e1601f729edd6d4a9dafa2b574f34192ab
--- /dev/null
+++ b/build/mac/edit_xibs.sh
@@ -0,0 +1,19 @@
+#!/bin/sh
+
+# Copyright (c) 2012 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.
+
+# This script is a convenience to run GYP for /src/chrome/chrome_nibs.gyp
+# with the Xcode generator (as you likely use ninja). Documentation:
+#   http://dev.chromium.org/developers/design-documents/mac-xib-files
+
+set -e
+
+RELSRC=$(dirname "$0")/../..
+SRC=$(cd "$RELSRC" && pwd)
+export PYTHONPATH="$PYTHONPATH:$SRC/build"
+export GYP_GENERATORS=xcode
+"$SRC/tools/gyp/gyp" -I"$SRC/build/common.gypi" "$SRC/chrome/chrome_nibs.gyp"
+echo "You can now edit XIB files in Xcode using:"
+echo "  $SRC/chrome/chrome_nibs.xcodeproj"