One Scaffold layout to rule them all

Expanded the existing CustomMultiChildLayout to handle all of the Scaffold's children.

This change also eliminates the FAB input dead-zone.

Remove use of FakeAsync in signing_test.dart

Make all the *GestureRecognizer classes inherit from a class called
GestureRecognizer. Give the old GestureRecognizer a name that is more
precise about its purpose. Remove the members of GestureArenaMember that
aren't used by GestureArenas.

Fixes navigation back to main menu in demo game

Check platform before opening /dev/urandom

- change how we expose settings at the RenderObject layer so that it's
  easier to maintain.
- expose the Widget owner chain in the RenderObject layer debug output
- add debug info to RenderOpacity, RenderIgnorePointer, RenderListener
- make the output for text nodes prettier

This means we no longer depend on the dart 1.13 beta SDK.

Add a test for signing/verifying with a random generated key pair.

Widen the dropdown menu item InkWells

Previously the horizontal padding on each menu item didn't response to taps.

Fixes #1916

Adding RawImage component

Previously, we'd leave the old values in the parent data if the types matches,
but not all render objects would reset these values during layout. For example,
RenderProxyBox doesn't set the position field because it doesn't read the
position field. However, leaving the old data there violates the invariants of
the box protocol and can cause trouble (e.g., localToGlobal giving the wrong
result).

Fixes #1939

Document the Gesture Detector constants.

Remove kEdgeSlop since it's obsolete on Android.

Add a test that verifies that when the first tap is canceled, the second
tap can become the first tap of a subsequent two-tap sequence.

Bare bones Mojo reimplementation of Dart’s http package

Add support for "extended height" toolbars - ToolBar bottom

Added a Widget-valued bottom ToolBar property. If set the bottom widget appears below the normal toolbar.

Removed the kToolBarHeight hardwiring from Scaffold.

Updated the "swipe away" demo to use an extended height toolbar.

Stocks list doesn't update when stocks are loaded

Previously, we passed widgets up the hierarchy to display them in the overlay,
but that breaks the change propagation logic because those widgets won't get
rebuilt.  Now we pass WidgetBuilders instead, which can be rebuilt when the
overlay rebuilds.

Fixes #1913

Tap and the othes classes in tap.dart are not really related.

Make RRect immutable and replace RRect.setRectXY with a fromRectXY co…

Add tests for flx Bundle.

Also cleaned up the flx code a bit. Replaced custom KeyPair class with
cipher's AsymmetricKeyPair.

Adds FlexJustifyContent.collapse