Radio<T> and ValueChanged<T>

This changes flutter's updater package to depend on and use the new flx
package. I also did a little cleanup of the Bundle interface.

Make Radio widgets take a type that describes the type of their value,
so that you can catch when you use the wrong value.

Standardise on ValueChanged<Foo> instead of having a FooValueChanged
for every value of Foo.

Keep the selection within bounds in deleteSurroundingText

This makes skyanalyzer also check the examples, and fixes everything it
found there.

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.

Working node to physics world conversions

Teleportation for physics bodies works again

Hide the keyboard in a deferred task.

Delete OldTextPainter

Also, delete all references to DOM APIs.

This is intended to eliminate the flicker that occurs when one widget hides
the keyboard and then another shows it within the same update cycle.

Fix #1471 Add double tap gesture

Now we use one velocity tracker per pointer.

Path by Kris Giesing.

Fixes #1704

The NewTextPainter is still disabled by default. A future patch will flip the
flag to enable it.

This patch uses a new approach to writing bindings by encoding data into array
buffers. This approach is more efficient than the existing IDL based approach.
If this works out well, we should convert our other performance-sensitive
interfaces to this approach in future patches.

Now AssetImage will look in the default bundle if you don't supply an explicit
bundle.

Fixes #680

Add .gitignore file to the flx package

Use FractionalOffset alignment value in Stack and IndexedStack

The magic for updating DebugSize's knowledge about whether it was
allowed to be used by the parent or not previously failed to correctly
deal with sizedByParent.

Unmatched heroes don't need to animate

Fix slow sound init in piano sample app

Fixes #1696. Also fixes handles leaking on error.

By default now heroes won't animate if they don't find a matching hero
on the target page. You can make them animate by setting 'alwaysAnimate:
true' on the Hero constructor.