Adds tests for type checks/casts for pkg:js types that inherit each
other. Similarly, adds subtyping tests for both when types are made
live and when they are not.
Also adds extends_test to lib_2.
Change-Id: Ia3ca03ad9c0291092998d1d3e98d672711fcf8e6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/205602
Commit-Queue: Srujan Gaddam <srujzs@google.com>
Reviewed-by: Riley Porter <rileyporter@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>
f7c284b9f0