Tweaks to DART_HOME (dart-lang#45)

Tweaks to DART_HOME (dart-lang#45)

Prepare for v1.2 release (dart-lang#41)

Add a way to setup raw builds for all channels (dart-lang#36)

* Add a way to setup raw builds for all channels

This allow developers reying on SDK changes to land and test changes
in other packages using a raw dev build so we can validate changes,
publish them, and roll to dependent packages (such as flutter) faster.

Closes: dart-lang#29

* Addressed CR comments

v1

Fix issue with pub global on Windows

Support CPU architectures

Support main channel

Support specific versions

Initial pre-release