dart2js cps: Add instruction for null checks.

pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.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.optimization.statement_rewriter;
 
 import 'optimization.dart' show Pass;
 import '../tree_ir_nodes.dart';
 import '../../io/source_information.dart';
 import '../../elements/elements.dart';
@@ skipping 1167 matching lines @@
     node.input = visitExpression(node.input);
     return node;
   }
 
   @override
   Statement visitYield(Yield node) {
     node.input = visitExpression(node.input);
     node.next = visitStatement(node.next);
     return node;
   }
+
+  @override
+  Statement visitNullCheck(NullCheck node) {
+    inEmptyEnvironment(() {
+      node.next = visitStatement(node.next);
+    });
+    if (node.condition != null) {
+      inEmptyEnvironment(() {
+        // Value occurs in conditional context.
+        node.value = visitExpression(node.value);
+      });
+      node.condition = visitExpression(node.condition);
+    } else {
+      node.value = visitExpression(node.value);
+    }
+    return node;
+  }
 }
 
 /// Result of combining two expressions, with the potential for reverting the
 /// combination.
 ///
 /// Reverting a combination is done by calling [uncombine]. In this case,
 /// both the original expressions should remain in the tree, and the [combined]
 /// expression should be orphaned.
 ///
 /// Explicitly reverting a combination is necessary to maintain variable
@@ skipping 73 matching lines @@
   VariableUseCallback callback;
 
   VariableUseVisitor(this.callback);
 
   visitVariableUse(VariableUse use) => callback(use);
 
   static void visit(Expression node, VariableUseCallback callback) {
     new VariableUseVisitor(callback).visitExpression(node);
   }
 }