* Run flutter tests in preview-dart-2 mode on travis flutter builds.

* Run dart2 tests on osx. Run dart tests in dart2.

* Fix name camelCase

* Default options to empty array, rather than null

* Troubleshoot failures

* More logging

* Troubleshoot: run single test

* Troubleshoot: run 15 tests

* Troubleshoot: run 15 tests with fix

* Try limit concurrency to 1

* Limit concurrency for preview-dart-2 tests to 4

* Move dart2 tests to allow_failures section

* Reinstate tests_dart_2 shard

* Raise concurrency to 8

* Reuse compiler across multiple test runs

* Allow to switch entry points when recompiling.

Actually use single compiler to incrementally recompile all tests executed by 'flutter test'.

* Remove leftover commented code

* Fix comment

* Lints