[mini_types] Introduce `_PreType` class hierarchy.

The `_PreType` class (and its subclasses) mirrors the `Type` class,
except that it represents types in a more raw form, before identifiers
have been resolved to their associated meanings. For example, the
`_PreType` representing `int` is a `_PrePrimaryType` whose `typeName`
field is the string `int`, whereas the `Type` representing `int` is a
`PrimaryType` whose `nameInfo` field points to the `TypeNameInfo`
object representing the class `int`.

Parsing of `Type` objects is now a two-step process: the string is
first converted to a `_PreType`, and then the `_PreType` is
materialized into a `Type` by looking up each identifier in it in the
`TypeRegistry`.

This will be needed in a follow-up CL that introduces support for
generic function types, to support the possiblity that a generic
function type's return type refers to one of its type parameters
(e.g. `List<T> Function<T>()`). The reason this is will be needed is
because the meaning of the return type (`List<T>`) can't be determined
until the `TypeParameter` object representing the type parameter `T`
has been created.

Change-Id: I9f4a73bdc0f38380518a9c39db8c788234adb806
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/395686
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>