Fix handling of null/bottom in front_end type inference.

pkg/front_end/testcases/inference/list_literals.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;
 
 test1() {
   var /*@type=List<int>*/ x = /*@typeArgs=int*/ [1, 2, 3];
   /*@promotedType=none*/ x.add(/*error:ARGUMENT_TYPE_NOT_ASSIGNABLE*/ 'hi');
   /*@promotedType=none*/ x.add(/*error:ARGUMENT_TYPE_NOT_ASSIGNABLE*/ 4.0);
   /*@promotedType=none*/ x.add(4);
   List<num> y = /*@promotedType=none*/ x;
 }
 
 test2() {
   var /*@type=List<num>*/ x = /*@typeArgs=num*/ [1, 2.0, 3];
   /*@promotedType=none*/ x.add(/*error:ARGUMENT_TYPE_NOT_ASSIGNABLE*/ 'hi');
   /*@promotedType=none*/ x.add(4.0);
   List<int> y = /*info:ASSIGNMENT_CAST*/ /*@promotedType=none*/ x;
 }
+
+main() {
+  test1();
+  test2();
+}