When compiling libraries, this feature extracts the minimal information
from the directives/pipes/modules of the library into `.ngsummary.json` files,
so that applications that use this library only need to be recompiled
if one of the summary files change, but not on every change
of the libraries (e.g. one of the templates).
Only works if individual codegen for libraries is enabled,
see the `generateCodeForLibraries: false` option.
Closes#12787
- `NgHost` to `CompilerHost`
- `AotCompilerHost.resolveFileToImport` to `AotCompilerHost.fileNameToModuleName`
- `AotCompilerHoset.resolveImportToFile` to `AotCompilerHost.moduleNameToFileName`
This is needed to resolve symbols without `.d.ts` files.
This bumps the version of the metadata from 1 to 2.
This adds logic into `ng_host.ts` to automatically upgrade
version 1 to version 2 metadata by adding the exported symbols
from the `.d.ts` file.
