refactor(@angular-devkit/core): add missing function return types

[clydin]

Adding explicit type information for function return types is needed to allow the @angular-devkit/core package to eventually be built with the isolatedDeclarations option.

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}