This package contains WidgetTester, which is very useful when writing tests for
widgets.

- force the time dilation to 1.0 for the Widget tests, so that a local
  change doesn't break all the tests during development.
- add missing license block to all the files.
- set ui.window.onBeginFrame to null when you use WidgetTester, so that
  the engine doesn't trigger any confusing frames after our fake frames.

Fixes #1084

Now that we've ported all the widget tests to fn3, we don't need a separate fn3
directory.

This argument isn't needed anymore now that State has a getter for context.

This patch is part of a sequence of patches towards fewer top-level libraries.
In this patch, the gesture libraries are combined into one gestures.dart
library.

Also:

 - don't mark a node as from the new generation if it is dirty, since we
   know it still has to be built.

 - establish the rule that you can't call setState() during initState()
   or build().

 - make syncChild() return early for unchanged children.

 - update the tests, including adding a new one.

Rename Outter to Outer.

Change _sync() in StatefulComponent to assert the argument type in the
signature rather than in the code.

Allows a non-initialised stateful component to be used as a source of
settings more than once. Instead of asserting that it was only being
used once, we assert that once you are stateful you don't get used as a
bag of settings, which is the real thing we were trying to avoid.

A lot of code ends up using StatefulComponents as a source multiple
times in a row, and before this would fail.

Patch by Ian Hickson