Implement matchInferableParameter in MiniAstOperations.

Previously the implementation of this method was a stub.

It turns out that all the necessary infrastructure was in place
already, however the type arguments supplied by MiniAstOperations to
TypeAnalyzerOperationsMixin and TypeAnalyzerOperations needed to be
changed: in the "mini_ast" representation of types, an
InferableParameter is represented by a String, not a
PromotedTypeVariableType. This is because InferableParameter is meant
to represent the declaration of the type parameter
(StructuralParameter for the CFE, TypeParameterElement for the
analyzer), not the type itself. The types used for unit testing in
_fe_analyzer_shared don't have a separate notion of the declaration of
a type parameter, so we just use its name.

Implementing this logic required adding a method
`TypeSystem.matchTypeParameterType`, which checks if a Type is a type
parameter type, and returns the name of the type parameter if so. I
based this on the previously existing `TypeSystem._isTypeVar` method
(which performed the same job but did not return the type parameter
name).

I also took the liberty of fixing a flow analysis test that treated
`T` as a type variable but failed to mark it as a type variable by
calling `addTypeVariable`.

This should help pave the way for unit testing more of the shared
infrastructure for types.

Change-Id: Ia7a9777ec3d90a5886567dcb9f831e388e372f32
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/386607
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>