dart2js cps: Add instruction for null checks.

pkg/compiler/lib/src/tree_ir/tree_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 tree_ir_tracer;
 
 import 'dart:async' show EventSink;
 import '../tracer.dart';
 import 'tree_ir_nodes.dart';
 
@@ skipping 164 matching lines @@
   }
 
   visitForeignStatement(ForeignStatement node) {
     _addStatement(node);
   }
 
   visitYield(Yield node) {
     _addStatement(node);
     visitStatement(node.next);
   }
+
+  visitNullCheck(NullCheck node) {
+    _addStatement(node);
+    visitStatement(node.next);
+  }
 }
 
 class TreeTracer extends TracerUtil with StatementVisitor {
   String get passName => null;
 
   final EventSink<String> output;
 
   TreeTracer(this.output);
 
   List<Variable> parameters;
@@ skipping 144 matching lines @@
   @override
   visitForeignStatement(ForeignStatement node) {
     printStatement(null, 'foreign ${node.codeTemplate.source}');
   }
 
   @override
   visitYield(Yield node) {
     String name = node.hasStar ? 'yield*' : 'yield';
     printStatement(null, '$name ${expr(node.input)}');
   }
+
+  @override
+  visitNullCheck(NullCheck node) {
+    printStatement(null, 'NullCheck ${expr(node.value)}');
+  }
 }
 
 class SubexpressionVisitor extends ExpressionVisitor<String> {
   Names names;
 
   SubexpressionVisitor(this.names);
 
   String visitVariableUse(VariableUse node) {
     return names.varName(node.variable);
   }
@@ skipping 251 matching lines @@
       String prefix = v.element == null ? 'v' : '${v.element.name}_';
       while (name == null || _usedNames.contains(name)) {
         name = "$prefix${_counter++}";
       }
       _names[v] = name;
       _usedNames.add(name);
     }
     return name;
   }
 }