Move Material page animations to Material layer.

PageRoute is now MaterialPageRoute.

This also changes the following:

- Now the HeroController is a Navigator observer, rather than a feature
  of HeroPageRoutes, which are gone. This means heroes can work between
  any kind of ModalRoute now.

- ModalPageRoute is moved from modal_barrier.dart to routes.dart.

- It allows routes to opt-out of their modal barrier being a shortcut to
  popping the route.

- Features of PageRoute that aren't Material-specific get promoted to
  ModalRoute features: storage, the subtree key, offstageness...

The AnimatedModalBarrier is still a ModalRoute feature.

More resilient Widget tests

- 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.

Fix quotes error in update flutter bash script

Give the StockSymbolView's Hero a Key

The StockSymbolBottomSheet includes a Hero, as does the StockSymbolPage and the StockRow. When two or more heroes with the same tag are gathered together Heros.of() expects them to have distinguishing non-null keys. Now they do.

Fixes #382

Safely clear SnackBack and BottomSheet placeholders

Correct one stocks demo bug: displaying the stock symbol viewer (single tap on a row) while a BottomSheet or SnackBar was up, would assert.

Thep SnackBack and BottomSheet placeholder keys are cleared when they're obscured by a route with an opaque OverlayEntry. So in that case there's no need to try and clear the placeholder's child again.

Updates for Windows development

Since we don't care about the port, just allow any characters between the spaces after the device ID and the product.

Running Windows 10, running "adb devices -l" with my device attached to the computer gives:
FA34MW904146           device product:cm_m7 model:One device:m7

Which wasn't being matched correctly (no info on USB port).

Need to change working directories so that the command works when called from outside the flutter directory.

Removed accidental debug code.

You should be able to nest Scaffolds

Prior to this patch, MultiChildLayoutDelegate couldn't be re-entered because it
cleared _idToChild when unwinding its stack. Now we restore the previous value
of _idToChild when we unwind.

Update four callers of Message constructor to pass in required arguments

Add an example for launching a URL

Bump dependencies on sky_engine and sky_services

Cursor is tiny when there's no text in an input

Now we use a zero-width space to force the engine to resolve the font and tell
us how large the text is likely to be once there's text in the widget.

Fixes #302

Fix tapcancel on slop.

Hit Testing Cleanup

More detailed debugging info for RenderPointerListener et al

Trivial whitespace fixes.

Make sure to send tapcancel when the primary pointer fails because of
slop, even if the gesture won by default.

Also, minor cleanup and clarification of an invariant.

Elaborate on the debugging information exposed by
RenderPointerListener, Listener, and GestureDetector.

- Drop the unused BindingHitTestEntry class. (Well, it was
  constructed, but its member was never used, so it wasn't doing
  anything a boring old HitTestEntry couldn't already do.)

- Add toString()s to HitTestEntry and HitTestResult, to aid in
  debugging hit-test related code.

Explicitly dispose heavy C++ objects

This patch calls dispose() explicitly on a number of heavy C++ objects so that
they can be eagerly finalized instead of waiting for GC.

Parent data not updated when reparenting using global keys

When we reactivate a subtree that had a global key, we weren't updating the
parent data because:

1) The child wasn't in the tree when we updated the parent data element.
2) The activated child didn't go through mount (just through update).

This patch moves the parent data update work to when we attach the render
object, which we do both during mount and when reactivating a child.

Fixes #345

Fix Dialog and PopupMenu completion logic

It's now (just) in TransitionRoute.

Added smoke tests for showMenu() and showDialog().

Fixes #337