Port test logic to Dart.

Add a CONTRIBUTING.md

This document describes the workflow for contributing code to this repository.

support older android devices (>= 17)

Teach flutter init how to depend on flutter using a relative path

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