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

add a flutter.yaml file for the widgets project

Make the analyzer actually work.

Turns out that ignoring all error lines that match the empty string is a
poor way to go.

Also, we have to update all the example packages now too, since we
analyze them. So just have travis use our update script.

Also, remove flutter_tools' old travis stuff. It's now part of a bigger
repo.

Also, make travis use the dev Dart SDK, since we need the new analyzer.
Stable is way too out of date, e.g. it still complains about libraries
not having names and mixins using 'super', and the strong mode hints are
even more aggressive than on dev.

Delete analyzer temporary directories

Remove constructor tear-offs

Some of our workflows have no way to silence analyzer warnings, and the
analyzer doesn't yet support constructor tear-offs, so pretend that we
can't use them for now.

flutter analyze command