Consider moving off of package:stager

[mossmana]

After pulling from the latest master of devtools, I received the following error:

Because devtools_app depends on integration_test from sdk which depends on test_api 0.7.4, test_api 0.7.4 is required.
And because test 1.25.8 depends on test_api 0.7.3, test 1.25.8 is forbidden.
And because devtools_tool depends on test ^1.25.8 and no versions of test match >1.25.8 <1.25.9, test ^1.25.9 is required.
And because test >=1.25.9 depends on analyzer >=6.0.0 <8.0.0 and stager 1.0.1 depends on analyzer ^5.1.0, stager 1.0.1 is forbidden.
Because no versions of stager match >1.0.1 <2.0.0 and devtools_app depends on stager ^1.0.1, stager 1.0.1 is required.
Thus, version solving failed.

The workaround is to add the following dependency_overrides to the top level pubspec.yaml:

dependency_overrides:
  analyzer: 6.11.0
  dart_style: 2.3.3

The longer term fix will require stager to update it's dependencies:
bryanoltman/stager#48

We should make a decision if we should continue to depend on stager going forward if the versioning issue cannot be addressed from the stager side.

[kenzieschmoll]

Once Widget Previews are available, we should be able to migrate off of the stager package and use Previews for the current set of scenes. CC @bkonyi this could be a good real-app use case for beta testing previews.