(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.

Add support for the appbar behavior described in the "Flexible space with image" section of https://www.google.com/design/spec/patterns/scrolling-techniques.html#scrolling-techniques-scrolling.

This patch adds ShaderLayer and makes ShaderMask use it.

Fixes #1155

Previously we used a positional argument for widgets that had multiple
children. Now we use a named argument that defaults to an empty list.

Fixes #241

ScrollableList2 is complete and subsumes all the use cases for the original
ScrollableList.

This patch also changed ScrollableList2 to use an Iterable instead of an
List for its children. This change lets clients map their underlying
data lazily. If the clients actually have a concrete list, we skip the
extra copy and grab the child list directly.

Now with fewer lists.

Also, fill in a back arrow when we can go back.

Fixes #699

This patch restructures how we handle drawer. The drawer is now a child of the
Scaffold, which wraps the Drawer in a DrawerController. The DrawerController
manages the interaction with the navigator as well as the edge swiping. The
DrawerController's state machine is driven almost entirely off its Performance,
which it now owns completely.

Fixes #90
Fixes #187
Fixes #192
Fixes #194
Fixes #604

This seems useful for numbering the entries in the list, for example.

Fixes #328

The name of the parameter wasn't consistent with IconButton or DrawerItem, etc.

Fixes #1871

MixedViewport didn't use the building:true flag when locking itself, so
when it caused a rebuild of its children, we assumed that nobody was
allowed to mark things dirty below the list, and things crashed when
Inherited people did in fact rebuild.

Also:
 - default offset for MixedViewport
 - don't bother rebuilding if the underlying RenderObject is going to
   rebuild anyway for some reason
 - better docs for the "items must have keys" assert
 - keep the FlipComponent stuff together in test_widgets.dart

Material design icons are defined to work at specific sizes: 18, 24, 36, 48.
The current API doesn't reflect that and just takes a size int. If an invalid
size is chosen an error is printed to the console and no icon shows up.

Fixes #1816

Instread of an explicit 'enabled' bool, this uses the presence of the
event handler to determine if a widget is enabled or not. This means
that if you've not passed a handler, your widget will be disabled, which
makes sense, since it wouldn't work anyway.

Adds this feature to checkbox, and ports raised button, flat button, and
radio buttons to this new model.

Adds a checkbox to card_collection that can be disabled.

Hide a (basically bogus) hint from the (soon to be disabled) strong hint
mode in the analyzer that this reveals.

Also:
 - give card_collection an option to turn on or off the edit widgets
 - give card_collection an option to control text alignment (when not editing)
 - give card_collection a "dump" option to aid debugging
 - make the gesture detector report which gestures it is listening to

Make Radio widgets take a type that describes the type of their value,
so that you can catch when you use the wrong value.

Standardise on ValueChanged<Foo> instead of having a FooValueChanged
for every value of Foo.

This makes skyanalyzer also check the examples, and fixes everything it
found there.

Now you don't need to pass the navigator around everywhere.

The style we use for callbacks in widgets is "onFoo". These classes were using
an order naming convention and just called their callbacks "callback".

It was confusing as to whether these were fractions or pixels.

Fixes #1622

MaterialApp assumes that you're using material design.

Also move radial reaction and fix imports for stats box.

These are now part of material.dart.

Previously, RenderObjectElements didn't support being marked dirty. This
is fine, except for MixedViewport and HomogeneousViewport, which have
builder functions to which they hand themselves as a BuildContext. If
those builder functions call, e.g., Theme.of(), then when the theme
changes, the Inherited logic tries to tell the RenderObjectElement
object that its dependencies changed and that doesn't go down well.

This patch fixes this by making RenderObjectElement a BuildableElement,
and making MixedViewport and HomogeneousViewport hook into that to
rebuild themselves appropriately.

Also, this was only found at all because ThemeData didn't implement
operator==, so we were aggressively marking the entire tree dirty all
the time. That's fixed here too.

Also, I changed card_collection.dart to have more features to make this
easier to test. This found bugs #1524, #1522, #1528, #1529, #1530, #1531.

Rather than using a custom render object, we can just use a Stack.

(These are changes cherry-picked from in-flight branches since they are
more independent and could be helpful even without those changes.)

- Change RouteBuilder's signature to take a single argument in which the
  other fields are placed, so that we can keep iterating on those
  arguments without having to break compatibility each time. Also, this
  makes defining route builders much simpler (only one argument to
  ignore rather than a variable number).

- Expose the next performance to RouteBuilders, since sometimes the
  route itself might not be where it's used.

- Allow BuildContext to be used to walk children, just like it can for
  ancestors

- Allow BuildContext to be used to get the Widget of the current
  BuildContext

- Allow StatefulComponentElement to be referenced with a type
  specialisation so that you don't have to cast when you know what the
  type you're dealing with actually is.

This patch converts drawer to using the "openDialog" pattern for managing its
state. Currently, the drawer entrance and exit animation aren't integrated with
the navigator's animation system because the drawer's animations can be stopped
and reversed, which the navigator can't yet understand. That means dismissing
the drawer via the system back button causes the drawer to be removed
instanteously.

Fixes #715
Fixes #1187

The ShaderMask widget enables rendering its child with an alpha channel defined by a Shader. For example if the Shader was a linear gradient in alpha then the component behind the ShaderMask's child would appear wherever the gradient's alpha value was not fully opaque.

The card_collection.dart example demonstrates this. Select the "Let the sun shine" checkbox in the app's drawer.

This argument isn't needed anymore now that State has a getter for context.

Initial snap offset support for ScrollableWidgetList (and ScrollableList<T>) and ScrollableMixedWidgetList. If a ```toSnapOffset(scrollOffset)``` function is provided, fling Scrolls will coast to the returned value. If ```alignmentOffset``` is specified then fling scrolls conclude when toSnapOffset's value lines up with the Scrollable widget's origin + alignmentOffset. For example if the Scrollable widget's height was 200.0, and alignmentOffset:100.0 was specified, then fling scrolls would end with the value returned by toSnapOffset() lined up with the center of the Scrollable.

This approach to Scrollable snapping assumes that the layout of whatever the Scrollable contains is known at the outset. This is often true however a ScrollableMixedWidgetList may not know its items' sizes until they've been reached by scrolling.

This is a first cut at snapping support. Among the things that remain to be done:
- Scrolling limits trump snapping. Snapping should probably trump scrolling limits.
- Drag scrolls aren't snapped. This may be desirable so perhaps the feature should be controlled with a flag.
- Specifying alignmentOffset as a percentage would probably be more convenient.
- It would be nice if one could wrap items in a SnapOffset value like: ```new SnapOffset(0.5, child: myItem)``` to snap to the center of the item.

Updated the CardCollection example: snapping and fixed size items can be turned on/off with Drawer checkboxes.