After this patch, if you try to use a widget that depends on being enclosed in
a material, you now get an assert and a debugPrint if you're not inside a
material.

Fixes #243

This feature can be used to implement password fields, where the user doesn't
want the text displayed on screen. Currently all the characters are hidden.
Eventualy we'll want to show the most recent character for some timeout.

Fixes #636

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.

This concludes the fn3 port!

If the text is empty, the IME may call deleteSurroundingText(1, 0) if the user
hits the delete key.  The selection should remain at position 0 if this
happens.

Also add an input widget API for the keyboard submit button

Add type annotations in many places.
Fix some identifiers to have more lint-satisfying names.
Make all operator==s consistent in style.
Reorder some functions for consistency.
Make ParentData no longer dynamic, and fix all the code around that.

These are now part of material.dart.

Fixes #1084

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

This updates to mojo 4e4d51ce28a and mojo sdk 711a0bcfb141b4 and updates the sky
package's pubspec.yaml dependency to '>=0.1.0 <0.2.0' to be compatible with
the current mojo package. This includes an update to the Mojo Dart generator to
produce real classes for enums and the corresponding updates for users of the
KeyboardType enum in Sky as well as one scoped_ptr->std::unique_ptr in shell
corresponding to a change in the Mojo EDK.

When a new version of the sky and sky_services package are pushed this will fix
domokit/mojo#440.

Previously EditableText would render a text widget with no cursor if the text
value was empty.

Also adjust the height of the cursor widget to reflect the style's line
height, and update the cursor painting to match.

We were just missing a few type casts in tests to make the analyzer happy.

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.