[Fuchsia] Build only the parts of the SDK that are needed

In particular, dart2js, dartdoc, and ddc aren't used.

R=johnmccutchan@google.com, pylaligand@google.com

Review-Url: https://codereview.chromium.org/2826793002 .

tools/write_revision_file.py

@@ -0,0 +1,34 @@
+#!/usr/bin/env python
+# Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+# for details. All rights reserved. Use of this source code is governed by a
+# BSD-style license that can be found in the LICENSE file.
+
+import argparse
+import os
+import sys
+import utils
+
+def ParseArgs(args):
+  args = args[1:]
+  parser = argparse.ArgumentParser(
+      description='A script to write the version string to a file')
+
+  parser.add_argument('--output', '-o',
+      type=str,
+      required=True,
+      help='File to write')
+
+  return parser.parse_args(args)
+
+
+def Main(argv):
+  args = ParseArgs(argv)
+  revision = utils.GetGitRevision()
+  if revision is not None:
+    with open(args.output, 'w') as f:
+      f.write('%s\n' % revision)
+  return 0
+
+
+if __name__ == '__main__':
+  sys.exit(Main(sys.argv))