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.

Now that there's a new string_scanner in town, we're having dependency
resolution conflict because of flutter_markdown's tight dependency. This
patch loosens the dependency and resolves the conflict.

We need to pin the version of package:analyzer we use to avoid version skew
within our project.

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.

What's important about this code is that it's presenting services outside the
VM, not the particular technology used to present the services.