We were setting a negative velocity tolerance, which never triggered because we
assume tolerances are positive numbers. Now we use a positive tolerance and
assert to catch this case in the future.

Fixes #2765

Fixes #2662

This patch adds a consistent, simple description for the common pattern of
having a single child widget.

The engine now provides the keyboard and raw keyboard services via the view
services. This patch updates the framework to look there for the keyboard.
Also, this patch migrates callers to the new MojoServices name for these entry
points.

- Make the textStyle optional
- Remove routes
- Make onGenerateRoute required
- Add some dartdocs

Now that the overlay actually respects the position we give it, put it
somewhere where the keyboard won't overlap it.

Noticed in #2733

The previous name was confusingly similar to AnimatedWidget, which is
for explicit animations.

The problem was we were using a tap gesture to stop the motion of the
drawer and a drag gesture to settle it. That can cause a broken
lifecycle. Now we use a single drag recognizer to drive the whole
lifecycle.

Fixes #775
Fixes #1276

Fixes #2708

The old text was misleading because it was written when RichText was an
implementation detail of Text.

Fixes #2624

This prepares us for depending on the LTR/RTL inherited property in
RenderObjectWidgets.

* justifyContent -> mainAxisAlignment
* alignItems -> crossAxisAlignment
* FlexJustifyContent -> MainAxisAlignment
* FlexAlignItems -> CrossAxisAlignment

Fixes #231

Removes an LTR bias in DismissDirection.

Fixes #2562

Fixes #2448

Fixes #2353

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

Fixes #2233

Fixes #1482

Fixes #1382

Fixes #774

Fixes #581

And fix the zillion issues that uncovered.

This reverts commit f41b3411, reversing
changes made to e33d8d96.

This was a bad check-in due to my mangling uploading a new version of the branch from a different machine.

This reverts https://github.com/flutter/flutter/pull/2639 and will be replaced by https://github.com/flutter/flutter/pull/2640

And fix the zillion issues that uncovered.

Instead of trying to flush the detached children from the child list, we keep
the set of detached children up to date and query on every read.

If there's not enough height for the given aspect ratio, we now try to see if
using the height to infer the width will fit the given constraints. The
algorithm we use is similar to the one we use for RenderImage.

Fixes #2620