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

Index: sdk/lib/_internal/compiler/implementation/ssa/tracer.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/ssa/tracer.dart	(revision 29323)
+++ sdk/lib/_internal/compiler/implementation/ssa/tracer.dart	(working copy)
@@ -263,6 +263,9 @@
   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';
   }