dart2js cps: Add instruction for null checks.

pkg/compiler/lib/src/tree_ir/optimization/pull_into_initializers.dart

 // Copyright (c) 2015, 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.pull_into_initializers;
 
 import 'optimization.dart' show Pass;
 import '../tree_ir_nodes.dart';
 
 /// Where a variable has been assigned.
@@ skipping 150 matching lines @@
   }
 
   Statement visitFor(For node) {
     return node;
   }
 
   Statement visitTry(Try node) {
     return node;
   }
 
+  Statement visitNullCheck(NullCheck node) {
+    if (node.condition != null) {
+      node.condition = visitExpression(node.condition);
+      // The value occurs in conditional context, so don't pull from that.
+    } else {
+      node.value = visitExpression(node.value);
+    }
+    return node;
+  }
+
   Expression visitAssign(Assign node) {
     bool inImpureContext = impureCounter > 0;
     bool inBranch = branchCounter > 0;
 
     // Remember the number of impure expression seen yet, so we can tell if
     // there are any impure expressions on the right-hand side.
     int impureBefore = impureCounter;
     int assignmentsBefore = assignCounter;
     node.value = visitExpression(node.value);
     bool rightHandSideIsImpure = (impureCounter > impureBefore);
@@ skipping 175 matching lines @@
 
   @override
   Expression visitForeignExpression(ForeignExpression node) {
     rewriteList(node.arguments);
     if (node.nativeBehavior.sideEffects.hasSideEffects()) {
       ++impureCounter;
     }
     return node;
   }
 }