There's no build time dependency from @angular/core to @angular/compiler,
so core can't directly refer to compiler types. To overcome this limitation,
there's a facade layer defined in the compiler and duplicated in core,
such that during runtime all types will correctly align.
There's a testcase in the compiler that verifies that all such facade types
are compatible across core and compiler, such that the core types can't get
misaligned with the actual definitions in the compiler. This suite of
tests were missing the `R3BaseMetadataFacade` facade type, so it was possible
for this type to get out of sync.
PR Close#31210
Currently with ViewEngine, if someone runs the platform's
`bootstrapModule` method in order to boostrap a module in
JIT mode, external component resources are properly resolved
*automatically*.
Currently with Ivy, the developer would need to manually call
`resolveComponentResources` in order to asynchronously fetch
the determined external component resources. In order to make
this backwards compatible with ViewEngine, and also since
platforms can already specify a `ResourceLoader` compiler
provider, we need to automatically resolve all external
component resources on module bootstrap.
--
Since the `ResourceLoader` is part of the `@angular/compiler`,
because ViewEngine performed the factory creation in the compiler,
we can't access the `ResourceLoader` token from within core.
In order to workaround this without introducing a breaking change,
we just proxy the `ResourceLoader` token to `core` through the
compiler facade. In the future, we should be able to move the
`ResourceLoader` to core when ViewEngine code no longer exists in
the `@angular/compiler`.
PR Close#29083
JoostK
Paul Gschwendtner
Miško Hevery