Remove test dependency from package:flutter

[ios] generate flx on first ios launch

fix the zip tool when building for ios

show stdout on xcode failures

Improve documentation for MultiChildLayoutDelegate

Reorder and document members in ScrollableState

Renames Weather demo

fix the stop command tests

The mojo_sdk has cleaned up its dependencies, so we no longer need to depend on
package:test from package:flutter.

Switch from hack_load_vm_file_hook to PlatformPlugin

Atom validation

refactor the stop command to not use DeviceStore

Add DismissDirection onDismissed() callback parameter

refactor to move an ios specific class out of globals.dart

Don't install the Cloud SDK on Travis

This patch switches us from using our previous hacking approach to integrating
with package:test to using the new PlatformPlugin interface.

Add a test for icon opacity crash

Add type parameters to uses of PopupMenuItem

We don't need this anymore.

Fixes #2361

Update engine.version

This test causes the buildbot to hang. We don't currently have any hang
protection, which means this test ends up killing the bot. Disable the
test for now why we work on the root issues.

[driver] serialize commands to plain strings

Update engine

This change brings in a new version of Mojo, which has improved Mozart
interfaces.

Turns out VM service only accepts flat parameter name/value pairs. It
only worked this far because vm_service_client (mistakenly) uses JSON
encoder to encode parameters.

This change separates the Command type hierarchy from that of Result and
tightens type constraints on Command object parameters.