[analyzer] Rework MockSdkElements to set enclosing elements earlier.

In the typical sequence of operations performed by the analyzer, an
element's chain of enclosing elements is guaranteed to be set before
that element is referred to by any `DartType`-derived objects. In a
follow-up CL, I will be adding some error checking to the unnamed
constructor of `InterfaceTypeImpl` that relies on this ordering
guarantee.

This change reworks `MockSdkElements` so that it also follows this
ordering guarantee. This will ensure that when the error checking is
added to `InterfaceTypeImpl`, it won't break tests that use
`MockSdkElements`.

This change has no visible effect on the analyzer behavior, since it
only touches test code.

Change-Id: Ibb60575578a8b3facb4014d3583eed23db2ff694
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/396521
Commit-Queue: Paul Berry <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>

Author: stereotype441