Draggable offsets and fix default drag feedback

GlobalKeys should preserve state across tree mutations

You can now set an offset that will be used for the hit testing for
drops. (It doesn't move the feedback by default; use a Transform for
that.) I also made the default feedback not be Opacity 0.5 always.

FractionallySizedBox

This patch causes widgets with global keys to drag their state (including their
children) with them as they travel through the element tree.

Moves from vector_math to vector_math_64

- Rename EdgeDims constructor to EdgeDims.TRBL().

- Add operator== to Size and Offset so that you can compare Size to
  DebugSize in checked mode.

- Add Size.lerp().

- Add various operators to EdgeDims. (*, /, ~/, %)

- Add EdgeDims.lerp().

- Update style guide. I went there to fix an EdgeDims constructor
  example, and stayed because some recent things came up and I wanted to
  add them before I forgot.

* Moves from vector_math to vector_math_64
* Adds support for Float64List in Dart bindings

Delay unmounting elements until the end of the frame

This patch prepares us to move these elements around in the tree.

Assert that the widget is the T in State<T>

Support for decoding images in lists

Heroes: Pass next Performance to route builders

Rename RouteState to StateRoute

Remove the mixed_viewport demo

Mostly because we can, but hopefully it'll catch someone who
accidentally copypasted their createState method.

This is step 1 in making it possible to have hero transitions between
routes. To make it possible for a route to have an "exit" animation when
a new route has been pushed on top of it, we provide the next route's
AnimationPerformance to the build function. It's null if there is no
next route or if the next route has no performance.

...for consistency with the other Route subclasses:
```
  dialog.dart:      class DialogRoute extends Route
  drag_target.dart: class DragRoute extends Route
  navigator.dart:   class PageRoute extends Route
  navigator.dart:   class StateRoute extends Route
  popup_menu.dart:  class MenuRoute extends Route
```

It's redundant with card_collection.dart, which does similar things but
much better.

This removes GlobalKey.currentElement in favour of GlobalKey.currentContext.

Fixes #1084

BoxConstraints trivial cleanup

Keep the two ConstrainedBox classes together

Add clamped simulation

Fix bounds checking error I introduced in previour cl, Address style …

Consolidate widget tests

Ensure that the item under the focal point stays in the same place despite zooming

@abarth