Gesture class hierarchy tune-up

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

Get more debug info into the rendering layer

- 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

Use crypto package for BASE64 encoding.

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

flx: Support for seeding the RNG for package signing.

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

flx: Remove deprecated KeyPair class.

Widen the dropdown menu item InkWells

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

Fixes #1916

Split big_red_button into two scripts:

Adding RawImage component

Always reset parentData when dropping children

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

Triple doubletap touch slop, add a doubletap test

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.

 - sky/tools/release_engine.py, which releases sky_engine and sky_services
 - sky/tools/release_packages.py, which releases flutter and flutter_sprites

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

Set the default linker run time search path on Mac and iOS