fix(core): refine Injector.get declaration to infer correct return type - #24151

fix(core): refine Injector.get declaration to infer correct return type

These changes address an overload resolution issue whereby Injector.get
calls would resolve to the deprecated overload, disabling all type
checks with an 'any' return type.

BREAKING CHANGE:
Applications may need to be corrected for revealed type errors.
Applications explicitly specifying the generic argument to the method
should remove the generic argument in favor of type inference.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] angular.io application / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Given x of type Type<T>|InjectionToken<T> and y of type U (excluding THROW_IF_NOT_NULL and undefined):

• injector.get(x, undefined) is any
• injector.get(x, THROW_IF_NOT_NULL) is any
• injector.get(x, y) is any
• injector.get(x, someCondition ? y : THROW_IF_NOT_NULL) is any

What is the new behavior?

• injector.get(x, undefined) is T
• injector.get(x, THROW_IF_NOT_NULL) is T
• injector.get(x, y) is T|U.
• injector.get(x, someCondition ? y : THROW_IF_NOT_NULL) is T|U

Does this PR introduce a breaking change?

[x] Yes
[ ] No

Any application injecting Injector may be affected. Applications may need to be corrected for revealed type errors. Applications explicitly specifying the generic argument to the method should remove the generic argument in favor of type inference.