Switch our pubspec.yamls to using SDK sources so that we can have consistent
source types when we depend on these packages from external packages using SDK
sources.

* Udpate to Dart `1.20.0-dev.1.0`.

Udpate to Dart `1.20.0-dev.1.0` and corresponding `analyzer` package.

* Added `--no-packages-dir`.

* Revert extension to master.

* Added ignores.

* Review nits.

* More nits.

* bump out dep on the test package

* add other pubspecs

The new version has an API change. Also, up our SDK requirements to be a
bit more realistic.

until we have a fix for the breaking changes

[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)