Change "mini types" type parameters to match CFE and analyzer.

Previously, the "mini types" representation used in
`_fe_analyzer_shared` unit tests represented unpromoted type
parameters using the `PrimaryType` class (which was also used for
interface types and special built-in types like `void`) and
represented promoted type parameters using a separate
`PromotedTypeVariableType` class.

This CL changes the "mini types" representation to use a single
`TypeParameterType` class for both unpromoted and promoted type
parameters.  This parallels the representation used by the analyzer
and CFE, so it should help pave the way for sharing type system logic
between the analyzer and CFE.

To allow the `Type` constructor to distinguish whether a given
identifier represents an interface type or a type variable, tests must
register all type names they will need, using either the static method
`TypeRegistry.addTypeParameter` or the static method
`TypeRegistry.addInterfaceTypeName`.

To prevent the type names registered by one unit test from interfering
with those registered by another, tests should call
`TypeRegistry.init` in a `setUp` callback and `TypeRegistry.uninit` in
a `tearDown` callback. Methods in `TypeRegistry` contain error checks
to help make sure these calls aren't forgotten.

Change-Id: I701842ad94899c819f1a059e660510a616d00456
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/387822
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>