Further prep work for #32525.
Further prep work for #32525.

This CL refactors the bodies of the following methods so that they can
be re-used by the summary linker:

- CompilationUnitElementImpl.getType
- LibraryElementImpl.createLoadLibraryFunction
- LibraryElementImpl.getImportsWithPrefix
- LibraryElementImpl.getType

Change-Id: I9b398d87323eb9ec6deb9a9a76bd6e9a1aa6bb76
Reviewed-on: https://dart-review.googlesource.com/48740
Reviewed-by: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@google.com>
Commit-Queue: Paul Berry <72d414e3a99fb120308437779d51667bcc0984eb@google.com>

dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk