Properly flatten FutureOr types when inferring the type of an async method.

pkg/front_end/test/fasta/strong.status

Index: pkg/front_end/test/fasta/strong.status
diff --git a/pkg/front_end/test/fasta/strong.status b/pkg/front_end/test/fasta/strong.status
index da91e45d99977b620970991cbbd94f7ea35a90e0..064c0773d80009621eb1370f82ee7dd4ba20a752 100644
--- a/pkg/front_end/test/fasta/strong.status
+++ b/pkg/front_end/test/fasta/strong.status
@@ -56,12 +56,6 @@ void-methods: Fail
 inference/constructors_too_many_positional_arguments: Fail
 inference/downwards_inference_annotations: Fail
 inference/downwards_inference_on_function_of_t_using_the_t: Fail
-inference/future_then_conditional: Fail
-inference/future_then_conditional_2: Fail
-inference/future_then_conditional_3: Fail
-inference/future_then_conditional_4: Fail
-inference/future_then_conditional_5: Fail
-inference/future_then_conditional_6: Fail
 inference/future_then_explicit_future: Fail
 inference/generic_functions_return_typedef: Fail
 inference/generic_methods_downwards_inference_fold: Fail