Clean up imports and exports.

Add an example of creating a custom RenderBox

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.

Cleanup MixedViewport

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

Flutter gallery dialog demos

ScrollableList was applying padding twice

We need to subtract the padding instead of adding it.

fix an NPE when the flutter.yaml file is missing

More BoxConstraints asserts

The asserting will continue until morale improves!

* Convert all assert(*.isNormalized) checks to use the new
  debugAssertIsNormalized technology.
* Convert CustomMultiChildLayout to use the new RenderingError
  technology to greatly improve the detail in errors you get when
  writing CustomMultiChildLayout delegates.
* Add BoxConstraints.copyWith().
* Indent the descendants in the rendering exception data dump so that
  when you have multiple children it's clearer what's going on.

ScrollBehavior should support negative min scroll offset

Fixes #1730

Refactor service-related code to be shared between android and IOS.

Improve asserts around BoxConstraints

The IOS code is totally untested. Need a Mac to test it.

Negative constraints never make sense, so catch those too.

Make RenderObject.layout's isNormalized assert use the newer more fancy
debug version of isNormalized.

Upgrade command shouldn’t validate project, fixes #1745

Teach PageableList about scroll anchors

Teach ScrollableList about scroll anchors

fix an issue with logging ios simulator output

Introduce globals

Tapping through drag targets.

Factor out the HitTestBehavior logic so that RenderMetaData can use it.

Use that in DragTarget.

ignore derived resources in ios/build

Add a refresh command that rebuilds the snapshot and reloads it on th…

Add scroll anchor for Block

This patch teaches block how to anchor its scrolling to the end rather than the
start.

Fixes #136

Draggable: childWhenDragging, maxSimultaneousDrags