This commit adds the basic building blocks for linking partial declarations.
In particular it provides a generic `FileLinker` class that delegates to
a set of (not yet implemented) `PartialLinker` classes.
The Babel plugin makes use of this `FileLinker` providing concrete classes
for `AstHost` and `AstFactory` that work with Babel AST. It can be created
with the following code:
```ts
const plugin = createEs2015LinkerPlugin({ /* options */ });
```
PR Close#39116
This commit adds the `AstHost` interface, along with implementations for
both Babel and TS.
It also implements the Babel vesion of the `AstFactory` interface, along
with a linker specific implementation of the `ImportGenerator` interface.
These classes will be used by the new "ng-linker" to transform prelinked
library code using a Babel plugin.
PR Close#38866