Remove redundant operator== and hashCode functions in card example.

It turns out that we aren't really using these. The identity logic is sufficient.

Also, add some asserts for a crash I had once but couldn't reproduce, in case that helps catch it next time.

... and move instructions to the wiki.

Extract and reuse common sky/shell components on Mac and iOS

Rationalise usage of keys in navigator.dart.

Route (named routes) no longer have a key, and have their own storage for their names.
RouseState no longer has a key, and uses an owner field pointing to a StatefulComponent instead.
As such, RouteBase no longer has a key.

HistoryEntry no longer uses a global int to ensure uniqueness.

Propagated this to stocks app.

Images don't appears in VariableHeightScrollables

When we build widgets during layout we weren't notifying those widgets that
they were mounted. Now we notify mount status changes when exiting build phases
inside layout.

Fixes #176

Separate width and height parameters for Image widgets

Fix analyzer warnings

GL bindings for "GL_KHR_blend_equation" and "GL_KHR_debug"

This change makes it easier to defined only the width or the height of an image
and let the other value be filled in from the image's intrinsic aspect ratio.

Fixes #175

Renamed AnimatedType to AnimatedValue

Updates sprite widget to handle changes to renderbox

Minor style changes around demo cards.

Make typography.white and typography.black constants.

Rename sky_packager to sky_snapshot

This program just creates a snapshot, not the whole package.

Dismissable: animate "snap back", only call dismiss() once

This demo is a fuzzer of sorts, testing in particular our text layout system and painting

Use didMount(), didUnmount() to manage the VariableHeightScrollable layoutState listener.