Set MethodInvocation.interfaceTarget during type inference.

pkg/front_end/testcases/inference/future_union_downwards_4.dart

Index: pkg/front_end/testcases/inference/future_union_downwards_4.dart
diff --git a/pkg/front_end/testcases/inference/future_union_downwards_4.dart b/pkg/front_end/testcases/inference/future_union_downwards_4.dart
index da6d18ac6f813b7414efd1bde1f1fbba9661cf98..ea093e25b7e2a3da7a3fc3da6cfed325bedc3c21 100644
--- a/pkg/front_end/testcases/inference/future_union_downwards_4.dart
+++ b/pkg/front_end/testcases/inference/future_union_downwards_4.dart
@@ -16,12 +16,12 @@ class MyFuture<T> implements Future<T> {
 
 Future f;
 // Instantiates Future<int>
-Future<int> t1 = f. /*@typeArgs=int*/ then(
+Future<int> t1 = f. /*@typeArgs=int*/ /*@target=Future::then*/ then(
     /*@returnType=MyFuture<int>*/ (/*@type=dynamic*/ _) =>
         new /*@typeArgs=int*/ MyFuture.value('hi'));
 
 // Instantiates List<int>
-Future<List<int>> t2 = f. /*@typeArgs=List<int>*/ then(
+Future<List<int>> t2 = f. /*@typeArgs=List<int>*/ /*@target=Future::then*/ then(
     /*@returnType=List<int>*/ (/*@type=dynamic*/ _) => /*@typeArgs=int*/ [3]);
 Future<List<int>> g2() async {
   return /*@typeArgs=int*/ [3];