Previously `DebugNode.classes` and `DebugNode.styles` mistakenly used an
object that is only *sometimes* a `StylingContext`. Also fixes a mistake
in `debug_node_spec.ts` where a test component was not declared in the
testing module.
There is still a bug here where `DebugNode` is not exposing *static*
values. This will need to be fixed in a follow up.
PR Close#28709