Add an alignment property to OverflowBox

This patch changes the default alignment of OverflowBox to center rather than
having a bias towards the upper-left corner.

Fixes #1236

fix two strong mode issues

add the engine revision to --version

Add support for asset variants; add resolution aware asset image management

make the output from create more terse

test command checks for test dir

make --device-id work with ios devices too

update description text for the trace command

Add a --full-restart flag to the start command that controls whether …

Add a --full-restart flag to the start command that controls whether we stop the application process

SkyActivity now allows reloading application Dart code within an existing
activity.  If a SkyActivity instance is already running, then passing
--no-full-restart will restart the Dart code without killing and restarting
the SkyShell application.
(full-restart will remain the default until the engine
that supports this is rolled out)

Also remove the obsolete --poke flag

Clean up shell.dart

Unify logging

rename to printTrace(), printStatus(), and printError()

tweaks to the ios code

The old style of import is deprecated.

Semantics

rename the init command to create

refactor the list command

examples/widgets/container.dart does not display the buttons

Allow collection of trace files when adbd is not running as root

Add a missing Material.

Fixes #1402

Also fix a bug where the trace command may capture the wrong file
if multiple trace file paths are in the Android log buffer.

Previously we found a lower bound timestamp for the trace path log
by running the date command on the device and parsing the result on
the host.  This could yield an inaccurate result if the device and
host are using different time zones.

The command will now obtain the most recent timestamp in the device's
time format by running logcat.

ignore the ios dir in examples

Resurrect spinning_mixed.

It had bit-rotted. Also improve our error reporting for missing Material.