Make it easier to run local engine tests using the Travis test script. (#7512)
Make it easier to run local engine tests using the Travis test script. (#7512)

Our test script is getting complicated enough that doing your own
testing with a custom engine has become tedious. Now you can just do:

```bash
(cd ~/dev/flutter/; FLUTTER_TEST_ARGS=--local-engine=host_debug_unopt dev/bots/test.sh)
```

...to run all your tests (assuming your flutter repo is in
`~/dev/flutter` and your engine in `~/dev/engine`).
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