FileToModuleHost aliasing supports compilation within environments that have
two properties:
1. A `FileToModuleHost` exists which defines canonical module names for any
given TS file.
2. Dependency restrictions exist which prevent the import of arbitrary files
even if such files are within the .d.ts transitive closure of a
compilation ("strictdeps").
In such an environment, generated imports can only go through import paths
which are already present in the user program. The aliasing system supports
the generation and consumption of such imports at runtime.
`FileToModuleHost` aliasing does not emit re-exports in .d.ts files. This
means that it's safe to rely on alias re-exports in generated .js code (they
are guaranteed to exist at runtime) but not in template type-checking code
(since TS will not be able to follow such imports). Therefore, non-aliased
imports should be used in template type-checking code.
This commit adds a `NoAliasing` flag to `ImportFlags` and sets it when
generating imports in template type-checking code. The testing environment
is also patched to support resolution of FileToModuleHost canonical paths
within the template type-checking program, enabling testing of this change.
PR Close#34649
cb11380515