Make PostExpectation generic.

[grouma]

An error cropping up while migrating tests to Dart 2.0 semantics is having invalid return types for thenAnswer which cause runtime issues.
For example consider the class Foo:

class Foo {
    Future<B> doSomething(A a);
}

Set up the mock for Foo as follows:

MockFoo foo = new MockFoo()
when(foo.doSomething(any(a)).thenAnswer((_) => new B())

The above will currently run without issue in the VM. However, the return type of thenAnswer is invalid as it should be returning Future<B> instead of the raw B value. I believe we can catch these errors through analysis. when uses PostExpectation which can be made generic. This will allow us to enforce the return type of thenAnswer accordingly.

[srawlins]

Sounds good. Maybe specifically Answering needs to be type annotated? https://github.com/dart-lang/mockito/blob/master/lib/src/mock.dart#L605

[grouma]

I'm not sure if that will help. We need to have a return type here: https://github.com/dart-lang/mockito/blob/master/lib/src/mock.dart#L320

[srawlins]

Ah I see. Do you see any way to type the T in PostExpectation<T>?

[grouma]

I think the type should be coming from here: https://github.com/dart-lang/mockito/blob/master/lib/src/mock.dart#L744

[srawlins]

Yeah I can see it. Sounds good, I'll accept a PR.

[srawlins]

Fixed with #101