Run dartfmt on package:js

BUG=
R=alanknight@google.com

Review-Url: https://codereview.chromium.org/2750123003 .

pkg/js/lib/src/varargs.dart

@@ -58,7 +58,6 @@ const _Rest rest = const _Rest();
 ///     foo.apply(null, [a, b].concat(others))
 ///
 dynamic spread(args) {
-  throw new StateError(
-      'The spread function cannot be called, '
+  throw new StateError('The spread function cannot be called, '
       'it should be compiled away.');
 }