Covers lib/ in package:flutter.

Fixes #7734

* More better toStrings and more vigorous smoke testing of gallery

* Update scroll_controller.dart

* Update sliver.dart

- ScrollableList
 - ScrollableLazyList
 - LazyBlock
 - MaterialList

Clients should use ListView instead.

I've been using these locally, but they're likely to be useful to other people
as well.

These functions were previously hidden inside other functions but they
are useful enough to expose directly. Specifically, I plan to use these
when implementing Scrollable2.ensureVisible.

The needsLayout getter is prone to misuse. See discussion on #3083.

Rumours have it that this is faster now.

Fixes https://github.com/flutter/flutter/issues/5154

Skia calls this BlendMode now and that's a better name.

Fixes #7200

LayoutCallback passes constraints to the callback, but the constraints
object has a different type for different subclasses. This lets you
call invokeLayoutCallback() with a specific type to verify that
everything is working as expected.

Other changes:

Slightly improve the error reporting in RenderObject.

Allow toStringShallow on RenderObject to have its separator configured.

More idiomatic use of constraints in performResize.

Trivial fixes to comments.

Make ProxyBox not use BoxParentData since it ignores the field.

Make applyPaintTransform more helpful if you use a different ParentData
subclass than RenderBox expects.

Make debugAssertIsValid actually fulfill its contract in RenderObject as
documented.

Add a childBefore for symmetry (we already had childAfter).

Fix the way we dump the child list when there's no children in a
multichild render object.

More asserts in the rendering test library.

Also fixes https://github.com/flutter/flutter/issues/6510

Also, update the docs to point to the correct property.

Fixes #6224

We weren't computing the bounds for composited transforms correctly. We
need to conjugate the transform by the offset in order to get the
correct paint bounds for the composited layer. We now also use the same
math in the non-composited case for consistency.

Also, don't scale the z-coordinate in RenderFittedBox.

Fixes #6293

The Skia team tells us that these go faster.

This match switches the framework to use the semantics backend in `dart:ui`
rather than the Mojo backend.

This patch adds `@checked` everywhere is needed to remove the
`strong_mode_invalid_method_override` strong mode error.

This change might make our grammar less perfect, but it will increase our
sanity.

This patch removes the Mozart dependencies from object.dart and
layer.dart, which will make it easier for us to move the Mozart
dependencies purely into the Fuchsia tree.

This rewrites imports of various mojom.dart files from the Flutter
engine repo to instead import normal-looking dart files from the
(new) flutter_services package. This package handles exporting the
correct symbols from generated code wherever that may live.

Includes an engine roll to 3551e7a48e2e336777b15c7637af92fd7605b6c5
which contains the new flutter_services package.

I did a pass through some of the code cleaning minor things up.

Previously, pumpWidget() would do a partial pump (it didn't trigger
Ticker callbacks or post-frame callbacks), and pump() would do a full
pump. This patch brings them closer together. It also makes runApp run a
full actual frame, rather than skipping the transient callback part of
the frame logic. Having "half-frames" in the system was confusing and
could lead to bugs where code expecting to run before the next layout
pass didn't because a "half-frame" ran first.

Also, make Tickers start ticking in the frame that they were started in,
if they were started during a frame. This means we no longer spin a
frame for t=0, we jump straight to the first actual frame.

Other changes in this patch:

* rename WidgetsBinding._runApp to WidgetsBinding.attachRootWidget, so
  that tests can use it to more accurately mock out runApp.

* allow loadStructuredData to return synchronously.

* make handleBeginFrame handle not being given a time stamp.

* make DataPipeImageProvider.loadAsync protected (rather than private),
  and document it. There wasn't really a reason for it to be private.

* fix ImageConfiguration.toString.

* introduce debugPrintBuildScope and debugPrintScheduleBuildForStacks,
  which can help debug problems with widgets getting marked as dirty but
  not cleaned.

* make debugPrintRebuildDirtyWidgets say "Building" the first time and
  "Rebuilding" the second, to make it clearer when a widget is first
  created. This makes debugging widget lifecycle issues much easier.

* make debugDumpApp more resilient.

* debugPrintStack now takes a label that is printed before the stack.

* improve the banner shown for debugPrintBeginFrameBanner.

* various and sundry documentation fixes

Fixes https://github.com/flutter/flutter/issues/5283

Other changes in this patch:

Rename OffStage to Offstage.
Fixes https://github.com/flutter/flutter/issues/5378

Add a lot of docs.

Some minor punctuation and whitespace fixes.

This patch includes a number of improvements:

 * Material page routes now put a repaint boundary inside their transition so
   they don't repaint during the transition.
 * Heroes that are on a quest now get a repaint boundary so we repaint them
   individually.
 * I've hoisted the transparent material for the product items up in the widget
   tree, which doesn't affect performance but makes the ink splashes reach the
   edge of the product cards.
 * I've changed the repaint rainbow visualization to make it easier to see
   what's going on.