[dynamic modules] Add pragmas to core library classes and members used to implement language features

This change adds pragmas which can be used in dart:* libraries to
specify classes and members which are used by the compiler
to implement certain language features. Compiler can insert
references to those classes and members when desugaring language
constructs, so dynamic module code can use these classes and members
implicitly even if they are private.

Previously, we required to specify these members in the
dynamic_interface.yaml before dynamic modules could use them.
However, this is cumbersome and error-prone.

Now, the following pragmas are recognized:

@pragma('dyn-module:language-impl:extendable')
@pragma('dyn-module:language-impl:can-be-overridden')
@pragma('dyn-module:language-impl:callable')

If dynamic interface yaml file is specified during compilation,
then classes/members annotated with these pragmas are treated as if
they are used in corresponding sections of dynamic_interface.yaml.

If dynamic interface yaml file is not specified, these pragmas
are ignored.

Bug: b/395992622

TEST=existing
CoreLibraryReviewExempt: no API changes, only adding pragmas.
Change-Id: I6606467fc5e5264a2565d7eb8e8baccc2eebea84
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/418663
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>