dart2js cps: Add instruction for null checks.

pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.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_nodes_sexpr;
 
 import '../constants/values.dart';
 import '../util/util.dart';
 import 'cps_ir_nodes.dart';
 import '../universe/call_structure.dart' show
@@ skipping 15 matching lines @@
   SExpressionStringifier([this.decorator]) {
     if (this.decorator == null) {
       this.decorator = (node, String s) => s;
     }
   }
 
   String access(Reference<Definition> r) {
     return decorator(r, namer.getName(r.definition));
   }
 
+  String optional(Reference<Definition> r) {
+    return r == null ? '()' : '(${access(r)})';
+  }
+
   String visitParameter(Parameter node) {
     return namer.nameParameter(node);
   }
 
   String visitMutableVariable(MutableVariable node) {
     return namer.nameMutableVariable(node);
   }
 
   /// Main entry point for creating a [String] from a [Node].  All recursive
   /// calls must go through this method.
@@ skipping 307 matching lines @@
   @override
   String visitYield(Yield node) {
     String value = access(node.input);
     return '(Yield $value)';
   }
 
   String visitRefinement(Refinement node) {
     String value = access(node.value);
     return '(Refinement $value ${node.type})';
   }
+
+  String visitNullCheck(NullCheck node) {
+    String value = access(node.value);
+    String condition = optional(node.condition);
+    return '(NullCheck $value $condition (${node.selector ?? ""}))';
+  }
 }
 
 class ConstantStringifier extends ConstantValueVisitor<String, Null> {
   // Some of these methods are unimplemented because we haven't had a need
   // to print such constants.  When printing is implemented, the corresponding
   // parsing support should be added to SExpressionUnstringifier.parseConstant
   // in the dart2js tests (currently in the file
   // tests/compiler/dart2js/backend_dart/sexpr_unstringifier.dart).
 
   String _failWith(ConstantValue constant) {
@@ skipping 94 matching lines @@
   void setReturnContinuation(Continuation node) {
     assert(!_names.containsKey(node) || _names[node] == 'return');
     _names[node] = 'return';
   }
 
   String getName(Node node) {
     if (!_names.containsKey(node)) return 'MISSING_NAME';
     return _names[node];
   }
 }