Fix RenderFractionallySizedBox's constraints logic

Fixes https://github.com/flutter/flutter/issues/2735

Provide APIs for exchanging messages with the host application

Update the engine

Fix Fast Fling Failure

Use the artifacts store for iOS assets

Update engine.version to get the ApplicationMessages interface

Dont differentiate between ios_arm and ios_x64 in flutter_tools

All artifacts for iOS are universal.

don't wait for the observatory if the app didn't start

Bump engine revision and ensure the Xcode project setup expects assets at the archive root.

Remove `analyze` tear-off regexp.

Scrollable sometimes pushes frames forever

Update engine.version

Fix _LazyWidgetProvider indexing, move Ali Connors

We were setting a negative velocity tolerance, which never triggered because we
assume tolerances are positive numbers. Now we use a positive tolerance and
assert to catch this case in the future.

Fixes #2765

Allow customizing of Switch thumbs

In the spirit of https://github.com/flutter/flutter/issues/1220, remove the tear-off regexp in favor of in-line suppressions (if needed).

Fixes #2621

Flexible AppBar with a TabBar

Improve error message when ChildView lacks a MediaQuery

refactor; fix apk warning

add support for a FLUTTER_DEV env var in the flutter script

Fixes #2662

Update FlutterPlugin.groovy

Provide more documentation for MojoShell

As usual, as I was doing this I ran into some stuff that seemed hard
to document as-is and so I changed it. In this case, in the "http"
library. The new code is more or less equivalent, I think, but the
resulting documentation makes it more obvious that it's wrong...

Refactor cassowary so it uses imports rather than parts.