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

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

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

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

Fixes #2430

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

Fixes #2198

Fixes #2361

This test causes the buildbot to hang. We don't currently have any hang
protection, which means this test ends up killing the bot. Disable the
test for now why we work on the root issues.

Fixes #2324

Makes it easier to use without a SizeObserver

Previously we would elide forward and reverse callbacks that canceled each
other out, which broke the expected state machine. Now we synchronously deliver
status callbacks when start an animation.

Fixes #1913

Now onScroll callbacks are always bracketed by onScrollStart and onScrollEnd
callbacks.

Fixes #1822

Also:
 - add operator==/hashCode/toString to ViewportDimensions
 - add toString to BindingBase
 - add toString and debugFillDescription to ScrollBehavior
 - fix a bug in the RawGestureDetectorState's replaceGestureRecognizers
 - rename MixedViewport's onExtentsUpdate to onExtentChanged
 - replace ExtentsUpdateCallback with ValueChanged<double>
 - remove a microtask for dispatching scroll start, since it
   did not appear to have any purpose
 - added dartdocs to Instrumentation until I understood it
 - made all event dispatch in Instrumentation drain microtasks

There's no reason to make clients supply a PointerRounter and a
GestureArena when constructing gesture recognizers. These objects are
statics and the gesture recognizers can just grab them directly.

Also, remove the callback constructor arguments. Almost no code used
them. Instead, people seem to prefer using the `..` operator to set
callbacks on the recognizers. Removing the arguments removes a bunch of
boilerplate.

Otherwise, changing the order of your children (e.g. shuffling a list of
children) will have no visible effect.

Currently the interface for recognizing gestures on text spans is pretty ugly,
but hopefully we can improve it with time.

Fixes #156

We forgot to compare the lengths of the lists.

One bug was masking the other, hence they both needed to be fixed
and tested separately.

@Hixie

Now we just have one TextSpan class that handles both simple strings, trees of
children, and styling both. This approach simplifies the interface for most
clients.

This patch also removes StyledText, which was weakly typed and tricky to use
correctly. The replacement is RichText, which is strongly typed and uses
TextSpan.

This helper makes it easier to write correct tests that involve
gestures.

Fixes #1855

We should mutate the status of the AnimationController before calling
the status callbacks so that we're prepared to be re-entered.

Fixes #1911

We were using an Offset, which represented pixels/second, but it wasn't
clear to clients whether that was pixels/ms. Now we use a Velocity class
that is explict about the units.

Fixes #1510
Fixes #785

See https://github.com/flutter/flutter/issues/1854

Our examples have been growing organically over time. This patch cleans
them up to illustrate specific aspects of Flutter.

Previously we had totally wrong behavior. Now we have more correct behavior and
testing.

Fixes #1808

Remove the SizeObserver you need to use snap alignment in lists by
having the lists provide the size themselves in the callback.

Also, remove snapAlignmentOffset since we couldn't figure out how to
explain it and it's not really needed.

I'm guessing the recent changes to the bindings fixed this crash.

Closes https://github.com/flutter/flutter/issues/248.

...even after winning the arena.

This lets it cooperate with other gestures like tap.

The way I implemented this was to refactor the entire Draggable gesture
logic to use a new kind of gesture detector called
MultiDragGestureRecognizer. It works a bit like
MultiTapGestureRecognizer but for drags.

Also some tweaks to the velocity estimator.

(see https://github.com/flutter/flutter/issues/1586)

Each layer is supposed to reexport the parts of the previous layer
that are part of its API.

- In painting.dart, export from dart:ui all the Canvas-related APIs
  that make sense to be used at higher levels, e.g. PaintingStyle.

- Delete painting/shadows.dart. It was dead code.

- In rendering/object.dart, export all of painting.dart.

- In widgets/basic.dart, export all of painting.dart and
  animation.dart. Some classes in animation/ are renamed to make this
  less disruptive and confusing to the namespace.

- Split out Stocks back into an import model rather than a part model,
  so that it's easier to manage its dependencies on a per-file basis.

- Move Ticker to scheduler library.

- Remove as many redundant imports as possible now.

- Some minor nit picking cleanup in various files.

This patch fixes a couple minor bugs and cleans up MixedViewport a bit.