Avoid use of run-as

Issue #1988 results from run-as not working on some phones,
because of Android issue 58373.

This change removes the need for run-as for running
"flutter start". It is still needed for tracing.

The FLX package should test itself

Rather than using the Flutter unit tests to test FLX, we can just test FLX
directly. Also, clean up the dependencies in the pubspec to match the code.

Update pubspecs and enable testing

This script runs the Flutter unit tests. By default, the script assumes you
have compiled a SkyShell in an "engine/src" that's a peer to the "flutter"
directory.

Using local paths ensures that each package sees each other package at HEAD.

add a flutter.yaml file to the init template

Add flutter.yaml for address_book example

Remove the --http option

Don't crash if no widthFactor is set in an unbounded context

We throw an exception if you tap the screen during load

Inside runApp, we were building the render tree for the app, but we weren't
calling layout, which meant we were running a hit test on a dirty tree. Now
hitTest specifically asserts that the tree is clean and we synchronously call
layout for your app during runApp.

Fixes #1960

Heroes don't reverse any more

We need to use the performance for the "from" route when going backwards. Also,
fix a bug in the HeroParty where it would call the quest finished callback
multiple times.

Fixes #1958

Adds showBottomSheet(), AlignTransition

Add a flag that selects which Android device ID is the target for Flu…

Remove scaffolding for Navigator1 to Navigator2 transition

Better asserts for MaterialApp and Navigator

These types belong at the MaterialApp level.

Fixes #1969

Teach Block about padding.

This patch paves the path for refactoring the Drawer into the Scaffold to
support edge swipes and peeks.

It's common to want a scrolling viewport but with padding around the
contents. Teaching Block about this makes the places that do this
substantially simpler and further buries ScrollableViewport and
BlockBody (they're now only used in scrollable.dart).

...and clean them up so it's easier to understand them.

Draggable is now itself a gesture arena member. This means it won't
conflict with other gesture recognisers in the same path.

This also allows variants of Draggable that are triggered by other
gestures.

Also, some cleanup of DoubleTapGestureRecognizer, GestureDetector, and
PrimaryPointerGestureRecognizer.

Also, make MultiTapGestureRecognizer support a timeout for longpress.

Also, make Draggable data be typed.

Also, hide warnings about constructor warnings for now. Analyzer doesn't
support them yet. (Have to do this on a per-line basis)

Directions for future research:
 - animating the avatar (enter/exit transitions)
 - interaction with the navigator (canceling a drag on page navigation, etc)
 - double-tap draggable