Fix type inference of factory constructor invocations.

pkg/front_end/testcases/inference/block_bodied_lambdas_lub.dart

 // Copyright (c) 2017, 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.
 
 /*@testedFeatures=inference*/
 library test;
 
 import 'dart:math' show Random;
 
 test2() {
   List<num> o;
   var /*@type=Iterable<num>*/ y =
       o. /*@typeArgs=num*/ /*@target=Iterable::map*/ map(
           /*@returnType=num*/ (/*@type=num*/ x) {
     if (new Random(). /*@target=dart.math::Random::nextBool*/ nextBool()) {
       return x. /*@target=num::toInt*/ toInt() /*@target=num::+*/ + 1;
     } else {
       return x. /*@target=num::toDouble*/ toDouble();
     }
   });
   Iterable<num> w = y;
   Iterable<int> z = /*info:ASSIGNMENT_CAST*/ y;
 }
+
+main() {}