@angular/core is unique in that it defines the Angular decorators
(@Component, @Directive, etc). Ordinarily ngtsc looks for imports
from @angular/core in order to identify these decorators. Clearly
within core itself, this strategy doesn't work.
Instead, a special constant ITS_JUST_ANGULAR is declared within a
known file in @angular/core. If ngtsc sees this constant it knows
core is being compiled and can ignore the imports when evaluating
decorators.
Additionally, when compiling decorators ngtsc will often write an
import to @angular/core for needed symbols. However @angular/core
cannot import itself. This change creates a module within core to
export all the symbols needed to compile it and adds intelligence
within ngtsc to write relative imports to that module, instead of
absolute imports to @angular/core.
PR Close#24677
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