More elaborate exceptions.

AnimatedOpacity

This provides an easy way to do implicit opacity transitions.

The elaboration will continue until morale improves.

Specific exceptions considered here:

 - size setter checking when you set the size
 - layout verifying that you do set the size
 - hitTest verifying that you aren't dirty and have a size
 - flex complaining about canFlex

Adds constants for FractionalOffsets

Adds some names for common FractionalOffset values.

Pointer events should pass buttons through

Fixes #2899

Remove two `flutter analyze` regular expressions

Update test.sh

fix an exception from flutter doctor

We can work around these in code rather than by post-processing the analyzer
output.

[driver] propagate exit code from package:test

Driver returns exit code 0 even when tests fail. This commit fixes it by
propagating the exit code set by `package:test`.

Use --packages rather than --package-root

The .packages file is replacing the packages directory of symlinks.

ChildView throws exceptions outside MojoShell

Rename debugOwner to debugCreator

Fixes #2940

`flutter upgrade` needed twice if sky_services dependencies change

Now we run `flutter --version` to update our local cache of packages before
running `pub upgrade`, which reads from that cache.

Fixes #2953

We're missing a null check.

Fixes #2949

Added a Form widget to manage multiple Input widgets.

Add all variants of the Roboto font as assets to Material Design apps

Improve _AnimatedEvaluation.toString

upgrade to test 0.12.13

Update to Dart SDK 1.15.0

...and fix a typo in a setState assert message.

toStrings for Animations and Animatables

RenderTable

Update engine

Add a Key to FlutterDemo widget

It's a good practice to let clients supply a key for every widget.

Fixes #2910