Added Grid row and column spacing, changed padding interpretation

Grid padding now defines the distance the overall grid is inset. The grid rowSpacing and columnSpacing attributes define the space between rows and columns respectively.

Add iOS configuration files for all examples and remove GN related files

Hide flutter run_mojo

Add iOS specific assets to the Stocks example

Remove flutter ios command

All the functionality of this command has been integrated into other commands.

Refactor: Rename initialize_xcode.dart to setup_xcodeproj.dart

Enforce that we get a final move to the pointer up location

`flutter start` initializes the Xcode project if the user has not already done so.

Previously we asserted that we got a pointer move to the location where the
pointer up occured, but not all sources of pointer packets respect that
invariant. Specifically, on the iOS simulator, of you drag outside the window,
you'll get a stream of pointers that violates that invariant.

This patch teaches the converter to insert a PointerMoveEvent to move the
pointer to the location where the up occurs, repairing the invariant.

Fixes #1912

Dart SDK is no longer needed

Add docs for RepaintBoundary

Tooling updates for dealing with native services distributed in pub packages

Fitness demo, initial version

flutter run should run pub get automatically

Improve the sample code for createState

This removes a step that can cause trouble.

Fixes #1904

The previous sample code isn't a common (or recommended) pattern.

Simplify the AutoLayout API

This object represents a rect the auto-layout system.

fixes found when running through the getting started process

This patch makes it easier to use the auto layout API:

* We no longer use operator== because that requires an ugly cast by the
  API user.
* Also, "leftEdge" is now just "left" for less verbosity.
* AutoLayoutChild not implies its key from the AutoLayoutParam object.
* We now correctly layout every child of a RenderAutoLayout object even
  if the solver doesn't flush any updates to that child.

Make update_dart_sdk.sh prettier and more resilient

Mention that cache directory is volatile.

Add support for autolayout to widgets

Internalize our dependency on the Dart SDK

This patch teaches the widget framework how to use Cassowary-based
autolayout. To integrate autolayout with widgets, I had to refactor how
RenderAutoLayout worked a bit. Now RenderAutoLayout follows the same
delegate pattern we use for custom paint and custom layout.