chore(pubspec): use perf_api v0.0.9

This fixes a Travis failure with Dart SDK 1.6.0.dev-8.0 that breaks
master.

  Chrome 37.0.2062 (Mac OS X 10.9.4) ERROR
    Internal error: 'package:perf_api/perf_api.dart': error: line 9 pos 29: expression is not a valid compile-time constant
      final Counters counters = new Counters();