Analyzer: Fix incorrect type in PropertyElementResolver._resolve.

The `unpromotedType` computed by `PropertyElementResolver._resolve`
(which eventually propagates to both `FlowAnalysis.propertyGet` and to
`PropertyElementResolverResult.getType`) is intended to represent the
static type of the "property get" expression in the absence of
promotion. If the property get targets a `PropertyAccessorElement` (a
getter), this is the getter's return type. But if it targets a
`MethodElement`, this is the function type of the method (because the
"property get" is doing a tear-off of the method).

Previously, `PropertyElementResolver._resolve` was always using the
return type, meaning that if the property get was targeting a
`MethodElement`, it was computing the wrong type. Fortunately, this
didn't lead to any bugs, because (a) the type passed to
`FlowAnalysis.propertyGet` is only used for determining whether a
field access should be promoted, and method tear-offs aren't
promotable, and (b) the analyzer only uses the type stored in
`PropertyElementResolverResult.getType` when the element being
targeted is a `PropertyAccessorElement`.

I'm in the middle of a larger arc of work trying to introduce a new,
simpler mechanism for flow analysis to be told about the static types
of property gets, and part of that arc of work will involve
introducing a temporary check to verify that the old and new
mechanisms see the same static types. Fixing this incorrect type will
allow the temporary check to pass.

Change-Id: Ib13b12d1176a781dc56c3e79c150df481b48a0d5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/390585
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

https://dart.googlesource.com/sdk/+/37ae078a3036578f9ae471d536384445a569cdf5