Null check remover understands JS fragments.

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

Index: pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
index 659b5e2f288f75b6f4e2d8ef46b682d026efa13b..c4b53d8e6d553afadd66d7fdf79c5a2d8619ec7b 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
@@ -18,7 +18,7 @@ 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 {
@@ -1545,6 +1545,15 @@ class ForeignCode extends UnsafePrimitive {
   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 {