This patch starts a semantic versioning scheme for Flutter. We're still
experimenting with semantic versioning, which means the version number
might not be reliable yet.

Also, add FractionalOffset.inscribe.

* Added custom radii to RRect.

This is the first commit towads an implementation of
MergeableMaterial. It adds custom radii to RRect.

* Renamed RRect constructors and added BorderRadius.

BorderRadius is a class similar to EdgeInsets that lets you define
all rounded corners of a rounded rectangle easily.

And make Scrollbar work with LazyBlock.

And an about box to the Stocks sample app.

Fixes https://github.com/flutter/flutter/issues/5086

BUG=https://github.com/flutter/flutter/issues/5103

These now have sorter names to make the callers less verbose.

These let you add Align widget to the inside or outside of a container.
Several customers have asked for these properties.

Fixes #4950

Some folks didn't realize these existed and asked us to add them. By
using them in examples, hopefully folks will discover them more easily.

* Adds `empty_statements` (new in `0.1.21`).
* (Re)enables `iterable_contains_unrelated_type` (fixed in linter#245).

This slipped in with an earlier commit.

- show the next error fully after a hot reload
- hide _AssertionError in stacks
- immediately rebuild after a reassemble, so that hit tests work
- catch errors when notifying global key listeners

This was a minor typo in the original code. It makes the license display
slightly cleaner.

Previously we would rebuild every route on the second animation frame to
rejigger the offstage bit and the animations. Now we build the page once and
update the offstage bit in place and update the animations using
ProxyAnimations.

DevFS asset support and deletion support

Update Dart (1.19.0-dev.0.0) and analyzer (0.27.4-alpha.19).

Also, add a few more debugging flags.

Fundamentally the core problem was that we were not saying how wide a
date picker should be. It should be 330 pixels, if I'm measuring the
spec's mocks correctly.

Fixes https://github.com/flutter/flutter/issues/4910