We still need this for the observatory build.

This reverts commit 5082974b.

The engine.git repo doesn't need a copy of the Dart SDK anymore.

Fixes https://github.com/flutter/flutter/issues/2618

* Updating CONTRIBUTING.md to mention latest paths

* Update CONTRIBUTING.md

Fixes #https://github.com/flutter/flutter/issues/3689

These commands are the same, but `run` is the new hotness.

Mention the waterfall and conventions around it.

Depends on https://github.com/flutter/website/pull/177

Sample code is not differentiated enough to disambiguate the full stop at the end of the line.

Bullet #3 was referencing "Android".

the user run download_android_tools.py before adding platform-tools
to their path so they don't fret over the platform-tools folder
not existing yet.