Fix some examples.

Add a Positioned constructor from a Rect

A common pattern is to use a Positioned with a Sized box to give both an offset
from the edge as well as a fixed size to the child. This patch rolls into into
the Stack layout algorithm for simplicity.

Fixes #250

Add all the icon sizes to the FLX

Add a --version option to flutter

Adding all the sizes of all the icons adds about 50 KB to the stocks FLX.
That's probably the right trade-off until we get better at pruning the set of
assets.

Fixes #235

This option just prints the current git revision.

Fixes #40

Remove redundant fetch functions and avoid crashing when no headers are set

Allow flutter engine path to be set when using run_mojo

Fix issue with forward-delete in editable text widget

Now that #371 is fixed, we no longer need to check logs for "sky"

- drag and drop was using the wrong draggables (long press vs short
  press).

- navigation.dart wasn't using material correctly and so was ending up
  with some red text.

- piano wasn't followig our style guide even a little.

- date picker was not scrollable.

Moar shebang.

Make it possible to run dev/update_packages.dart directly from the shell.

@abarth

Support for setting HTTP headers

Fix #373 Add abbreviation for flutter logs -c

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.