Remove insertRange.

Review URL: https://codereview.chromium.org//13956006

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@21464 260f80e4-7a28-3924-810f-c04153c831b5

lib/src/git_source.dart

@@ -148,7 +148,7 @@ class GitSource extends Source {
       // directory.
       ensureDir(to);
       var args = ["clone", from, to];
-      if (mirror) args.insertRange(1, 1, "--mirror");
+      if (mirror) args.insert(1, "--mirror");
       return git.run(args);
     }).then((result) => null);
   }