Export gridpaper.dart

Create new 'flx' package. Just a copy for now.

This will be shared by sky/updater and flutter_tools. Currently
bundle.dart and signing.dart are unmodified copies of the files from
updater and flutter_tools, respectively. This is so future changes are
easier to see.

This grid makes it significantly easier to track down errors when you're
looking at coordinates in a render tree dump and want to see if they're
what you expect or if they're wildly away from the right location.

These new PerformanceView classes are transforms over PerformanceViews.
The first is a cheap way to tell code that expects a performance to not
animate after all, and the other is a way to replace a view of a
performance going in one direction with a view of a performance going in
the other direction.

See also https://github.com/flutter/engine/issues/1708

A transition that rotates its contents.

Assert at build time that PageRoute route builders do not return null
widget trees.

Also very minor new code comments and code reorg to help make the heroes
patch easier to review.

(These are changes that are unrelated to Heroes but that were part of
the Heroes patch.)

UniqueKey

Sometimes you need a Key (not a GlobalKey) that is only equal to itself
but can be used multiple times in the tree. Enter UniqueKey!

The 0.2.1 release of the mojo package isn't actually compatible with Flutter
despite claiming to be a minor release. The real fix is to make Flutter
compatible with that release, but there are several yak shaves involved.

First pass on sprite physics node hierarchy integration

Fixes #1702

Adds EnterExitTransition widget

This widget smoothly resizes as its child changes and lets you provide entrance
and exit transitions for its children.

Close the UrlLoaderProxy on error.

RenderOffStage test

Also, give RenderViewport a default offset.

Fixes #1680.

I accidentally lost this when extracting RenderOverflowBox from
RenderProxyBox.

Introduces a new Widget that lays its child out, but does not render it.
Also, moves RenderOverflowBox to its own file since it has nothing to do
with proxy box really.

We call this function _getInnerConstraints in other classes. Also, this change
makes it private for RenderOverflowBox.

The style we use for callbacks in widgets is "onFoo". These classes were using
an order naming convention and just called their callbacks "callback".

Provide a way to schedule a callback for immediately after the current
frame has been sent to the GPU thread. This is useful for times where
you want to immediately schedule yourself for another build or layout
because, for instance, you just displayed frame zero of an animation and
you want to use the metrics from that layout to set up the actual
animation to begin immediately, such that the very next frame is frame 1.

Fix analyzer warnings in examples

Add support for verifying .flx signatures when updating

Adds a step to the updater to verify that the new .flx package is signed and
untampered.

Each .flx contains a signed manifest file. The manifest contains a SHA-256 hash
of the .flx contents. See bundle.dart for a description of the new .flx format.

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