Fix issue 12336: there was a bug when updating phis at exit of a block with a HTypeKnown instructio…

sdk/lib/_internal/compiler/implementation/ssa/tracer.dart

 // Copyright (c) 2013, 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 tracer;
 
 import 'dart:async' show EventSink;
 
 import 'ssa.dart';
 import '../js_backend/js_backend.dart';
@@ skipping 245 matching lines @@
       return "Continue ${node.label.labelName}: (B${target.id})";
     }
     return "Continue: (B${target.id})";
   }
 
   String visitDivide(HDivide node) => handleInvokeBinary(node, '/');
 
   String visitExit(HExit node) => "exit";
 
   String visitFieldGet(HFieldGet node) {
+    if (node.isNullCheck) {
+      return 'null check on ${temporaryId(node.receiver)}';
+    }
     String fieldName = node.element.name;
     return 'field get ${temporaryId(node.receiver)}.$fieldName';
   }
 
   String visitFieldSet(HFieldSet node) {
     String valueId = temporaryId(node.value);
     String fieldName = node.element.name;
     return 'field set ${temporaryId(node.receiver)}.$fieldName to $valueId';
   }
 
@@ skipping 236 matching lines @@
 
   String visitTypeKnown(HTypeKnown node) {
     assert(node.inputs.length == 1);
     return "TypeKnown: ${temporaryId(node.checkedInput)} is ${node.knownType}";
   }
 
   String visitRangeConversion(HRangeConversion node) {
     return "RangeConversion: ${node.checkedInput}";
   }
 }