fix #25944, improve Future.then inference

pkg/analyzer/test/generated/strong_mode_test.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.test.generated.strong_mode_test;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/src/dart/element/element.dart';
@@ skipping 103 matching lines @@
         type = returnExp.expression.staticType;
       }
       typeTest(type);
     }
 
     check("f0", _isFutureOfDynamic);
     check("f1", _isFutureOfDynamic);
     check("f2", _isFutureOfDynamic);
 
     check("f3", _isFutureOfInt);
-    // This should be int when we handle the implicit Future<T> | T union
-    // https://github.com/dart-lang/sdk/issues/25322
-    check("f4", _isFutureOfDynamic);
+    check("f4", _isFutureOfInt);
     check("f5", _isFutureOfInt);
 
     check("g0", _isFutureOfDynamic);
     check("g1", _isFutureOfDynamic);
     check("g2", _isFutureOfDynamic);
 
     check("g3", _isFutureOfInt);
-    // This should be int when we handle the implicit Future<T> | T union
-    // https://github.com/dart-lang/sdk/issues/25322
-    check("g4", _isFutureOfDynamic);
+    check("g4", _isFutureOfInt);
     check("g5", _isFutureOfInt);
   }
 
   void test_async_propagation() {
     String code = r'''
       import "dart:async";
 
       Future f0() => new Future.value(3);
       Future f1() async => new Future.value(3);
       Future f2() async => await new Future.value(3);
@@ skipping 27 matching lines @@
         type = returnExp.expression.staticType;
       }
       typeTest(type);
     }
 
     check("f0", _isFutureOfDynamic);
     check("f1", _isFutureOfDynamic);
     check("f2", _isFutureOfDynamic);
 
     check("f3", _isFutureOfInt);
-    // This should be int when we handle the implicit Future<T> | T union
-    // https://github.com/dart-lang/sdk/issues/25322
-    check("f4", _isFutureOfDynamic);
+    check("f4", _isFutureOfInt);
     check("f5", _isFutureOfInt);
 
     check("g0", _isFutureOfDynamic);
     check("g1", _isFutureOfDynamic);
     check("g2", _isFutureOfDynamic);
 
     check("g3", _isFutureOfInt);
-    // This should be int when we handle the implicit Future<T> | T union
-    // https://github.com/dart-lang/sdk/issues/25322
-    check("g4", _isFutureOfDynamic);
+    check("g4", _isFutureOfInt);
     check("g5", _isFutureOfInt);
   }
 
   void test_async_star_method_propagation() {
     String code = r'''
       import "dart:async";
       class A {
         Stream g0() async* { yield []; }
         Stream g1() async* { yield* new Stream(); }
 
@@ skipping 2101 matching lines @@
 main() {
   var v = x;
   v; // marker
 }
 int x = 3;
 ''';
     assertPropagatedAssignedType(code, typeProvider.intType, null);
     assertTypeOfMarkedExpression(code, typeProvider.intType, null);
   }
 }