Allow independent rendering pipelines

Replace EdgeInsets.TRBL with EdgeInsets.fromLTRB

Scrollbar thumb color, etc

This matches the pattern from Rect. I've left EdgeInsets.TRBL marked as
deprecated to give clients a chance to update.

Fixes #2860

Fixes #2723

several fixes to the doctor command

Improve docs for SizedBox

Fixes #2873

Make the border painting code reusable.

Add embedded font assets to the asset list

Inherited widgets should work across reactivations

Tweaks to Border to make it easier to subclass.

Any font asset referenced in the fonts section of flutter.yaml will now be
included in the FLX assets.  It's no longer necessary to separately list the
font in the assets section.

Previously we updated only the reactivated element's inherited widgets. Now we
update all the inherited widgets for the whole subtree that's being
reactivated. We need to visit the whole subtree anyway, so there isn't much of
a loss.

Fixes #2733
Fixes #2438
Fixes #1701

Fix minor typos (EdgeInsets, ImageResource)

Also, more docs. Also, expose isUniform.

An example showing how to send messages between the host app and Flutter

Remove wrong comment

Allow run_mojo command to run an app at a URL.

This test is no longer disabled.

Fixes #2827

filter some noisy messages from adb logcat

print the os from flutter doctor

re-work the doctor command

In the spirit of "you are what you measure", let's measure some more things...

Scrollable physics should be reasonable when sizes change

filesystem or not.

Fixes #2113

Previously, when the content extent changed during a scroll interaction, we'd
stop the current scroll interaction and reset the scroll offset. Now we try to
continue the scroll interaction (e.g., drag, fling, or overscroll) even through
the underlying scroll behavior has changed.

For physics-based scroll interactions, we keep the current position and
velocity and recompute the operative forces. For drag interactions, we keep the
current position and continue to let the user drag the scroll offset.

After this patch, we still disrupt non-physical scroll animations that are
operating outside the new scroll bounds because it's not clear how we can
sensibly modify them to work with the new scroll bounds.

Fixed a typo