This commit introduces the setClassMetadata() private function, which
adds metadata to a type in a way that can be accessed via Angular's
ReflectionCapabilities. Currently, it writes to static fields as if
the metadata being added was downleveled from decorators by tsickle.
The plan is for ngtsc to emit code which calls this function, passing
metadata on to the runtime for testing purposes. Calls to this function
would then be tree-shaken away for production bundles.
Testing strategy: proper operation of this function will be an integral
part of TestBed metadata overriding. Angular core tests will fail if this
is broken.
PR Close#26860
Alex Rickabaugh