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

Support hairline borders

Clean up prints in services/*

We really shouldn't have two separate network library wrappers... I filed https://github.com/flutter/flutter/issues/2889 on that. But for now, this makes one of them have a generally better error handling story, and makes the other one only dump to the console in debug mode. It also makes both use debugPrint instead of print, and makes one use the 'mojom' prefix for mojom imports.

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

Previously, border with '0' was ambiguous. Sometimes we treated it as
hairline borders, sometimes as "don't show the border", though even in
the latter case we did some graphics work sometimes. Now we have an
explicit BorderStyle.none flag to not draw the border efficiently.