Second attempt to fix "not found" error for type vars in bounds

[copybara-service[bot]]

Second attempt to fix "not found" error for type vars in bounds

First attempt was https://github.com/dart-lang/mockito/pull/671 and I had to roll it back, since it caused breakages in two ways:

1. Sometimes ParameterElement from type.parameters had enclosingElement set to null and we use that in one helper function. That was easy to fix, we could just pass methodElement to that function directly. It's probably correct that ParameterElement of a FunctionType doesn't link back to a MethodElement, but it's weird that sometimes it does, so it wasn't caught in the tests. I had to get rid of using type.parameters anyway because of the second problem.
2. type.parameters don't contain parameters' default values (totally correct, since default values belong to methods, not to types), but that means we can't use them, since we do need default values.

So I ended up with a more hacky solution, that uses type.typeFormals just to get correct references for method's type parameters, and then uses typeParameters, returnType and parameters for the rest as before.

Original commit description:

Use FunctionTypedElement.type while generating method overrides

Turns out FunctionTypedElement.typeParameters could be inconsistent for MethodMembers returned by InheritanceManager3.getMember2. FunctionTypedElement.type.typeFormals seem to be always good, but we have to also use type.parameters and type.returnType instead of just parameters and returnType in this case.

Fixes #658