Re-remove ad hoc Future.then inference.

pkg/front_end/testcases/inference/future_then_explicit_future.dart

Index: pkg/front_end/testcases/inference/future_then_explicit_future.dart
diff --git a/pkg/front_end/testcases/inference/future_then_explicit_future.dart b/pkg/front_end/testcases/inference/future_then_explicit_future.dart
index b1e71f896cded0910b40fc2e24cf8c2d35ad82c0..240d675c9c9b099106c25bb93aa884c5f6fe00d1 100644
--- a/pkg/front_end/testcases/inference/future_then_explicit_future.dart
+++ b/pkg/front_end/testcases/inference/future_then_explicit_future.dart
@@ -9,9 +9,9 @@ import "dart:async";
 
 m1() {
   Future<int> f;
-  var /*@type=Future<List<int>>*/ x = /*@promotedType=none*/ f
-      .then<Future<List<int>>>(/*error:ARGUMENT_TYPE_NOT_ASSIGNABLE*/
-          /*@returnType=List<int>*/ (/*@type=int*/ x) => /*@typeArgs=int*/ []);
+  var /*@type=Future<List<int>>*/ x = /*@promotedType=none*/ f.then<
+          Future<List<int>>>(/*error:ARGUMENT_TYPE_NOT_ASSIGNABLE*/
+      /*@returnType=List<dynamic>*/ (/*@type=int*/ x) => /*@typeArgs=dynamic*/ []);
   Future<List<int>> y = /*@promotedType=none*/ x;
 }