Declares a top-level getter that returns the outputs directory to which
Flutter Driver tests can write any output files. Timeline data defaults
to this directory.

*  Gallery demo start-time performance test

* fix analysis errors

* review comments; fix test

* re-add an export for debugPrint

Fixes https://github.com/flutter/flutter/issues/2713

[driver] utility for extracting and saving timeline summary

This commit contains:

- FlutterDriver API for e2e tests usable in conjunction with package:test
- FlutterDriverExtension to be enabled by the application in order to
  allow an external agent to connect to it and drive user interactions and
  probe into the element tree
- initial implementations of tap, findByValueKey and getText commands (to
  be expanded in future PRs)