fix #25944, improve Future.then inference

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

 // Copyright (c) 2015, 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.
 
 // TODO(jmesserly): this file needs to be refactored, it's a port from
 // package:dev_compiler's tests
 /// Tests for type inference.
 library analyzer.test.src.task.strong.inferred_type_test;
 
 import 'package:analyzer/dart/element/element.dart';
@@ skipping 882 matching lines @@
   l = (l = /*info:INFERRED_TYPE_LITERAL*/[1]);
 }
 ''');
   }
 
   void test_downwardsInferenceAsyncAwait() {
     checkFile('''
 import 'dart:async';
 Future test() async {
   dynamic d;
-  List<int> l0 = /*warning:DOWN_CAST_COMPOSITE should be pass*/await /*pass should be info:INFERRED_TYPE_LITERAL*/[d];
+  List<int> l0 = await /*info:INFERRED_TYPE_LITERAL*/[/*info:DYNAMIC_CAST*/d];
   List<int> l1 = await /*info:INFERRED_TYPE_ALLOCATION*/new Future.value(/*info:INFERRED_TYPE_LITERAL*/[/*info:DYNAMIC_CAST*/d]);
 }
 ''');
   }
 
   void test_downwardsInferenceForEach() {
     checkFile('''
 import 'dart:async';
 Future main() async {
   for(int x in /*info:INFERRED_TYPE_LITERAL*/[1, 2, 3]) {}
@@ skipping 602 matching lines @@
 int get y => null;
 ''');
     var x = mainUnit.types[0].fields[0];
     expect(x.type.toString(), 'int');
   }
 
   void test_futureThen() {
     checkFile('''
 import 'dart:async';
 Future f;
-Future<int> t1 = f.then((_) => new Future<int>.value(42));
+Future<int> t1 = f.then((_) async => await new Future<int>.value(3));
+Future<int> t2 = f.then(/*info:INFERRED_TYPE_CLOSURE*/(_) async {return await new Future<int>.value(3);});
+Future<int> t3 = f.then((_) async => 3);
+Future<int> t4 = f.then(/*info:INFERRED_TYPE_CLOSURE*/(_) async {return 3;});
+Future<int> t5 = f.then((_) => new Future<int>.value(3));
+Future<int> t6 = f.then((_) {return new Future<int>.value(3);});
+Future<int> t7 = f.then((_) async => new Future<int>.value(3));
+Future<int> t8 = f.then(/*info:INFERRED_TYPE_CLOSURE*/(_) async {return new Future<int>.value(3);});
 ''');
   }
 
+  void test_futureThen_conditional() {
+    checkFile('''
+import 'dart:async';
+Future<bool> f;
+Future<int> t1 = f.then((x) async => x ? 2 : await new Future<int>.value(3));
+Future<int> t2 = f.then(/*info:INFERRED_TYPE_CLOSURE*/(x) async {return await x ? 2 : new Future<int>.value(3);});
+Future<int> t5 = f.then((x) => x ? 2 : new Future<int>.value(3));
+Future<int> t6 = f.then((x) {return x ? 2 : new Future<int>.value(3);});
+''');
+  }
+
+  void test_futureUnion_asyncConditional() {
+    checkFile('''
+import 'dart:async';
+
+Future<int> g1(bool x) async { return x ? 42 : /*info:INFERRED_TYPE_ALLOCATION*/new Future.value(42); }
+Future<int> g2(bool x) async => x ? 42 : /*info:INFERRED_TYPE_ALLOCATION*/new Future.value(42);
+
+Future<int> g3(bool x) async {
+  var y = x ? 42 : /*info:INFERRED_TYPE_ALLOCATION*/new Future.value(42);
+  return y;
+}
+    ''');
+  }
+
+  void test_futureUnion_downwards() {
+    checkFile('''
+import 'dart:async';
+Future f;
+// Instantiates Future<int>
+Future<int> t1 = f.then((_) => /*info:INFERRED_TYPE_ALLOCATION*/new Future.value(/*error:ARGUMENT_TYPE_NOT_ASSIGNABLE*/'hi'));
+
+// Instantiates List<int>
+Future<List<int>> t2 = f.then((_) => /*info:INFERRED_TYPE_LITERAL*/[3]);
+Future<List<int>> g2() async { return /*info:INFERRED_TYPE_LITERAL*/[3]; }
+Future<List<int>> g3() async { return /*info:INFERRED_TYPE_ALLOCATION*/new Future.value(/*info:INFERRED_TYPE_LITERAL*/[3]); }
+''');
+  }
+
   void test_genericMethods_basicDownwardInference() {
     checkFile(r'''
 /*=T*/ f/*<S, T>*/(/*=S*/ s) => null;
 main() {
   String x = f(42);
   String y = (f)(42);
 }
 ''');
   }
 
@@ skipping 2761 matching lines @@
   }
 
   /// Adds a file using [helper.addFile] and calls [helper.check].
   ///
   /// Also returns the resolved compilation unit.
   @override
   CompilationUnitElement checkFile(String content) {
     return helper.checkFile(content).element;
   }
 }