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

build/module_args/mojo.gni

Index: build/module_args/mojo.gni
diff --git a/build/module_args/mojo.gni b/build/module_args/mojo.gni
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+# 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.
+
+# This variable should point to the parent directory of the Mojo SDK.
+mojo_sdk_root = "//"
+
+# To build the Mojo shell from source, set this variable to true. To use the
+# prebuilt shell, omit this variable or set it to false. Note that the prebuilt
+# shell will be used only on platforms for which it is published (currently
+# Linux and Android).
+mojo_build_mojo_shell_from_source = true
+
+# To build the network service from source, set this variable to true. To use
+# the prebuilt network service, omit this variable or set it to false.
+mojo_build_network_service_from_source = true