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

pkg/front_end/testcases/inference/future_then_conditional_6.dart

Index: pkg/front_end/testcases/inference/future_then_conditional_6.dart
diff --git a/pkg/front_end/testcases/inference/future_then_conditional_6.dart b/pkg/front_end/testcases/inference/future_then_conditional_6.dart
index eeee25fb3c029aeee68d8e14ef557573e5d3a76c..1cf97849ee767646fecd875f4ee0406721ee8ec8 100644
--- a/pkg/front_end/testcases/inference/future_then_conditional_6.dart
+++ b/pkg/front_end/testcases/inference/future_then_conditional_6.dart
@@ -14,7 +14,7 @@ class MyFuture<T> implements Future<T> {
   MyFuture<S> then<S>(FutureOr<S> f(T x), {Function onError}) => null;
 }
 
-void main() {
+void test() {
   Future<bool> f;
   Future<int> t1 = f. /*@typeArgs=int*/ /*@target=Future::then*/ then(
       /*@returnType=Future<int>*/ (/*@type=bool*/ x) async =>
@@ -32,3 +32,5 @@ void main() {
     return /*info:DOWN_CAST_COMPOSITE*/ x ? 2 : new Future<int>.value(3);
   });
 }
+
+main() {}