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.

I had to add a setLocale method to WidgetTester and
split the code in FlutterBinding which handled locale
changes to allow me to dispatch a locale change w/o actually
changing what the c++ code reports as the locale.

Also added the test to Travis.

@abarth @jason-simmons

Using local paths ensures that each package sees each other package at HEAD.

sky_engine is now at 0.0.45
sky_services is now at 0.0.45
flx is now at 0.0.4
flutter is now at 0.0.15
flutter_sprites is now at 0.0.12

sky_engine is now at 0.0.44
sky_services is now at 0.0.44
flx is now at 0.0.3
flutter is now at 0.0.14
flutter_sprites is now at 0.0.11

We can build Fitness again now that playfair has been updated to account for
the package:flutter rename.