Introduce a showPopupMenu() function

Instead of having to manage the popup menu from your app's build
function, you now just call showPopupMenu() with the menu's position and
it takes care of everything for you.

This solves the problem that the popup menu was trying to mutate the
state of the navigator from within its own initState() function.

Also, remove the "route" argument to RouteBase.build() since it equals
"this" by definition...

Also, remove ModalOverlay, and instead put that logic in the navigator.

Improvements to EffectLine in sprites

Move findAncestorRenderObjectWrapper to RenderObjectWrapper

Fix Build: Remove incorrect include of deleted picture_rasterizer.h

Remove picture_rasterize.h|cc

Fix Rect intersection; add test

That way it's closer to where it's used, and it's more obvious that
anyone can write such functions.

Allow route transitions to be more flexible

Update skia revision

- Fix AnimationTiming to have defaults for 'interval' and 'curve' since
  that seems to be how we use it.

- Merge RouteBase.build and RouteBase.buildTransition

- Get rid of HistoryEntry, since it added nothing

- Broke out RouteBase.createPerformance() so subclasses can change what
  is created.

- Build the routes backwards so that we more efficiently avoid building
  hidden routes.

- Introduce an explicit way (!hasContent) for RouteState to avoid
  building, rather than the implicit "build returns null" we had before.

In EditableText, wrap the placeholder text in a Row so it will expand to its parent's width

This will ensure that the width of an empty Input is consistent with the
width of an Input that contains text.

Also add a unit test for the Input widget and a way for tests to provide mock
implementations of Mojo services such as the keyboard.

SkySprites should import the public libraries

Adjust indent in Positioned to match style guide.

Require that you pass transitions a performance.

Importing the public libraries caused a name conflict with dart:sky because we
assume people will import dart:sky into a namespace, so I've also changed
skysprites to import dart:sky into a namespace.

Lift docs from Markdown to dartdoc

I've also removed the top-level description of the Sky package. Instead, we
should host that content on flutter.io.

This fixes #1103.

Provide hooks for when exceptions are thrown.

This might be helpful for #1219.

Also, remove inDebugMode since it's redundant with just using asserts,
which compile entirely out in release mode.

Move theme into material.dart

Also, introduce Colors and Typography to hold the material colors and the
typography declarations. Previously we expected clients of these libraries to
import them into a namespace, but that doesn't play nice with re-exporting them
from material.dart.

Handle the case of a Widget being moved down

When we sync() a Component, we need to clear the old Component's _child
pointer, otherwise if we reuse that Component we'll get confused about
what the old child is.

Improve debugging aids for widgets, rendering.

Kill sky_server correctly on Mac

Kill the sky_server on Mac in a way that works to avoid unexpected behavior when working with more than one app at once.