Null check remover understands JS fragments.

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

 // Copyright (c) 2013, 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;
 
 import 'dart:collection';
 import 'cps_fragment.dart' show CpsFragment;
 import '../constants/values.dart' as values;
 import '../dart_types.dart' show DartType, InterfaceType, TypeVariableType;
 import '../elements/elements.dart';
 import '../io/source_information.dart' show SourceInformation;
 import '../types/types.dart' show TypeMask;
 import '../universe/selector.dart' show Selector;
 
 import 'builtin_operator.dart';
 export 'builtin_operator.dart';
 
 // These imports are only used for the JavaScript specific nodes.  If we want to
 // support more than one native backend, we should probably create better
 // abstractions for native code and its type and effect system.
-import '../js/js.dart' as js show Template;
+import '../js/js.dart' as js show Template, isNullGuardOnFirstArgument;
 import '../native/native.dart' as native show NativeBehavior;
 
 abstract class Node {
   /// A pointer to the parent node. Is null until set by optimization passes.
   Node parent;
 
   /// Workaround for a slow Object.hashCode in the VM.
   static int _usedHashCodes = 0;
   final int hashCode = ++_usedHashCodes;
 
@@ skipping 1506 matching lines @@
       this.nativeBehavior, {this.dependency})
       : this.arguments = _referenceList(arguments);
 
   accept(Visitor visitor) => visitor.visitForeignCode(this);
 
   bool get hasValue => true;
 
   void setParentPointers() {
     _setParentsOnList(arguments, this);
   }
+
+  bool isNullGuardOnNullFirstArgument() {
+    if (arguments.length < 1) return false;
+    // TODO(sra): Fix NativeThrowBehavior to distinguish MAY from
+    // throws-nsm-on-null-followed-by-MAY and remove
+    // [isNullGuardForFirstArgument].
+    if (nativeBehavior.throwBehavior.isNullNSMGuard) return true;
+    return js.isNullGuardOnFirstArgument(codeTemplate);
+  }
 }
 
 class Constant extends Primitive {
   final values.ConstantValue value;
   final SourceInformation sourceInformation;
 
   Constant(this.value, {this.sourceInformation}) {
     assert(value != null);
   }
 
@@ skipping 1263 matching lines @@
     plug(new Branch.loose(_definitions.getCopy(node.condition),
         _copies[node.trueContinuation.definition],
         _copies[node.falseContinuation.definition])
       ..isStrictCheck = node.isStrictCheck);
   }
 
   visitUnreachable(Unreachable node) {
     plug(new Unreachable());
   }
 }