angular-cn/packages/compiler-cli/test/ngtsc/fake_core

fake_core is a library designed to expose some of the same symbols as @angular/core, without requiring compilation of the whole of @angular/core. This enables unit tests for the compiler to be written without incurring long rebuilds for every change.

• @angular/core is compiled with @angular/compiler-cli, and therefore has an implicit dependency on it. Therefore core must be rebuilt if the compiler changes.
• Tests for the compiler which intend to build code that depends on @angular/core must have a data dependency on @angular/core. Therefore core must be built to run the compiler tests, and thus rebuilt if the compiler changes.

This rebuild cycle is expensive and slow. fake_core avoids this by exposing a subset of the @angular/core API, which enables applications to be built by the ngtsc compiler without needing a full version of core present at compile time.