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

build/host_prebuilt_jar.gypi

+# 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.
+
+# This file is meant to be included into a target to provide a rule to
+# copy a prebuilt JAR for use on a host to the output directory.
+#
+# To use this, create a gyp target with the following form:
+# {
+#   'target_name': 'my_prebuilt_jar',
+#   'type': 'none',
+#   'variables': {
+#     'jar_path': 'path/to/prebuilt.jar',
+#   },
+#   'includes': [ 'path/to/this/gypi/file' ],
+# }
+#
+# Required variables:
+#   jar_path - The path to the prebuilt jar.
+
+{
+  'dependencies': [
+  ],
+  'variables': {
+    'dest_path': '<(PRODUCT_DIR)/lib.java/<(_target_name).jar',
+    'src_path': '<(jar_path)',
+  },
+  'all_dependent_settings': {
+    'variables': {
+      'input_jars_paths': [
+        '<(dest_path)',
+      ]
+    },
+  },
+  'actions': [
+    {
+      'action_name': 'copy_prebuilt_jar',
+      'message': 'Copy <(src_path) to <(dest_path)',
+      'inputs': [
+        '<(src_path)',
+      ],
+      'outputs': [
+        '<(dest_path)',
+      ],
+      'action': [
+        'python', '<(DEPTH)/build/cp.py', '<(src_path)', '<(dest_path)',
+      ],
+    }
+  ]
+}