anchor.diff:      return exists(path).chain((exists) {
         if (!exists) return _clone(_getUrl(id), path);
   
  -      return runProcess("git", ["pull", "--force"], workingDir: path,
  -          pipeStdout: true, pipeStderr: true).transform((result) {
  +      return runProcess("git", ["pull", "--force"], workingDir: path)
  +          .transform((result) {
           if (!result.success) throw 'Git failed.';
           return null;
         });
  
positive.code:    */
    Future<String> _revisionAt(PackageId id) {
      return runProcess("git", ["rev-parse", _getEffectiveRef(id)],
          workingDir: _repoCachePath(id), pipeStderr: true).transform((result) {
        if (!result.success) throw 'Git failed.';
        return result.stdout[0];
      });
  
negative.code:    * Clones the repo at the URI [from] to the path [to] on the local filesystem.
     */
    Future _clone(String from, String to) {
      return runProcess("git", ["clone", from, to]).transform((result) {
        if (!result.success) throw 'Git failed.';
        return null;
      });
  