Improve Future<T>.then<R> inference

pkg/analyzer/test/src/task/strong/inferred_type_test.dart

Index: pkg/analyzer/test/src/task/strong/inferred_type_test.dart
diff --git a/pkg/analyzer/test/src/task/strong/inferred_type_test.dart b/pkg/analyzer/test/src/task/strong/inferred_type_test.dart
index a4bc6b7a6edb0a56a50eb8084de9829cd19b5d69..513e6e938e079a607faea454013310cc99337e33 100644
--- a/pkg/analyzer/test/src/task/strong/inferred_type_test.dart
+++ b/pkg/analyzer/test/src/task/strong/inferred_type_test.dart
@@ -1268,6 +1268,14 @@ int get y => null;
     expect(x.type.toString(), 'int');
   }
 
+  void test_futureThen() {
+    checkFile('''
+import 'dart:async';
+Future f;
+Future<int> t1 = f.then((_) => new Future<int>.value(42));
+''');
+  }
+
   void test_genericMethods_basicDownwardInference() {
     checkFile(r'''
 /*=T*/ f/*<S, T>*/(/*=S*/ s) => null;