Transform applies its transform before painting, but sometimes you want
the widget to layout after its transform has been applied. We can't
handle general tranforms in this way because we can't couple width and
height constriants, but we can handle certain rotations.

Fixes #1214

We'll need this for RTL support because the RTL state will live in the widget
tree. Also, remove the `oldWidget` argument to updateRenderObject because there
aren't any clients for it.

Well, all the easy ones, anyway.

For some reason `// ignore:` isn't working for me so I've disabled
lints that need that. Also disabled those that require a ton of work
(which I'm doing, but not in this PR, to keep it reviewable).

This adds:
- avoid_init_to_null
- library_names
- package_api_docs
- package_names
- package_prefixed_library_names
- prefer_is_not_empty
- sort_constructors_first
- sort_unnamed_constructors_first
- unnecessary_getters_setters

Specifically:

* Handle null styles in TextSpan without crashing in toString().

* Handle null children in TextSpan child lists without crashing in
  toString().

* Handle entirely empty TextSpans in toString() explicitly.

* Assert that TextSpans don't contain nulls in various places. This is
  done more often than one might think necessary, because it turns out
  that TextSpan takes a (mutable) List for one of its arguments, so
  who knows what it will contain at any given time. By asserting all
  over the place, hopefully we'll catch it near the change if they do
  change it.

* Add a RichText example to Stocks to exercise RichText and TextSpans.

See also: https://github.com/flutter/flutter/issues/2514, https://github.com/flutter/flutter/issues/2519

This patch prepares us to pass a BuildContext to RenderObjectWidgets, which
will make it possible to rebuild RenderObjectElements:

 * Delay creation of the render object until mount(). That will let us pass
   `this` to createRenderObject and have the inherited elements be initialized.

 * Cleanup widgets that take builder closures to prepare for their
   RenderObjectElement to be rebuilt more often.

 * Add a test for the interaction between inherited widgets and
   MixedViewport.

Related to #2598

It's slightly more efficient.

And fix a zillion omissions this uncovered.

Also, resolve our package conflict, since reflectable has been fixed.

It has performance implications in release mode.

We'll enable the lint once https://github.com/dart-lang/linter/issues/195 is fixed.

After running a widget test, we now clear out the widget tree and check that we
didn't leak any timers or animations.

Also, fix several bugs that this addtional check revealed.

Fixes #2481

Fixes #2085

BUG=https://github.com/flutter/flutter/issues/2274

We need to apply the alignment to the source rect because the ouput rect fills
the destination rect.

Fixes #1514

The getSize function from MultiChildLayoutDelegate might depend on information
other than the incomming constraints.

Fixes #2077

This patch tweaks the definition of max intrinsic size to give implementations
more wiggle room to compute there intrinsic sizes more efficiently.
Specifically, implementations can now ignore the incomming constraint in the
cross axis if they prefer.

This patch removes an assert that is no longer valid w.r.t. the new definition
of intrinsic sizing.

Fixes #2099

This adds an assert to verify that we have already got a binding when we
get a semantics listener. Otherwise we crash. (I mean, we still crash,
but now in checked mode we crash on an assert instead. Progress?)

Mostly just adds some examples to match the examples with use for
similar concepts in dart:ui.

Fixes https://github.com/flutter/flutter/issues/2403

I have an e-mail ready to send to flutter-dev about this.

This makes WidgetError more like RenderingError, which will aid with https://github.com/flutter/flutter/issues/2356.

Fixes https://github.com/flutter/flutter/issues/2443

Fixes https://github.com/flutter/flutter/issues/1346

Previously we got confused and started sending start/end pairs for each tick of
the fling animation.

Fixes #2430

Fixes https://github.com/flutter/flutter/issues/2369

Fixes https://github.com/flutter/flutter/issues/2436

Putting this code in a separate library let's services that depend only on the
shell be clearer about their dependencies.