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

Update ISSUE_TEMPLATE.md

add a screenshot command

Fix Travis

The collections package changed to add some stricter type annotations.
We now pass those type annoations. I also sent
https://github.com/dart-lang/collection/pull/26 to make the type
annotations match what they were previously.

Add: flutter build ios [--[no-]simulator]

First pass at new Flutter gallery front page

Add a callback when a Draggable is dropped without being accepted.

This allows the user to build an iOS application bundle from the command line even if no device is attached to the workstation.

Bump engine revision to pick up iOS fixes.

Fix app bar padding without leading widget

If there's no leading widget, we just want 8.0 pixels of padding instead of all
the padding to get over to the 72.0 pixel key line.

Fixes #2912

Add a default MediaQuery value

Run cleanup

ImageFit.fitWidth/fitHeight and minor docs fixes

Now MediaQuery.of always returns a non-null value. By default, you get the
values associated with the current ui.Window.

Fixes #2894

Validate flutter.yaml against a JSON schema