dart2js cps: Add instruction for null checks.

pkg/compiler/lib/src/cps_ir/cps_ir_tracer.dart

 // Copyright (c) 2014, 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 dart2js.ir_tracer;
 
 import 'dart:async' show EventSink;
 import 'cps_ir_nodes.dart' as cps_ir;
 import '../tracer.dart';
 
@@ skipping 224 matching lines @@
     return '$name $value';
   }
 
   visitSetMutable(cps_ir.SetMutable node) {
     String variable = names.name(node.variable.definition);
     String value = formatReference(node.value);
     return 'SetMutable $variable := $value';
   }
 
   String formatReference(cps_ir.Reference ref) {
+    if (ref == null) return 'null';
     cps_ir.Definition target = ref.definition;
     if (target is cps_ir.Continuation && target.isReturnContinuation) {
       return "return"; // Do not generate a name for the return continuation
     } else {
       return names.name(ref.definition);
     }
   }
 
   visitConstant(cps_ir.Constant node) {
     return "Constant ${node.value.toStructuredString()}";
@@ skipping 127 matching lines @@
     String object = formatReference(node.object);
     String index = formatReference(node.index);
     String value = formatReference(node.value);
     return 'SetIndex $object $index $value';
   }
 
   visitRefinement(cps_ir.Refinement node) {
     String value = formatReference(node.value);
     return 'Refinement $value ${node.refineType}';
   }
+
+  visitNullCheck(cps_ir.NullCheck node) {
+    String value = formatReference(node.value);
+    String condition = formatReference(node.condition);
+    return 'NullCheck $value condition:$condition selector:${node.selector}';
+  }
 }
 
 /**
  * Invents (and remembers) names for Continuations, Parameters, etc.
  * The names must match the conventions used by IR Hydra, e.g.
  * Continuations and Functions must have names of form B### since they
  * are visualized as basic blocks.
  */
 class Names {
   final Map<Object, String> names = {};
@@ skipping 262 matching lines @@
     unexpectedNode(node);
   }
 
   visitYield(cps_ir.Yield node) {
     unexpectedNode(node);
   }
 
   visitRefinement(cps_ir.Refinement node) {
     unexpectedNode(node);
   }
+
+  visitNullCheck(cps_ir.NullCheck node) {
+    unexpectedNode(node);
+  }
 }