Null check remover understands JS fragments.

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

Index: pkg/compiler/lib/src/cps_ir/backward_null_check_remover.dart
diff --git a/pkg/compiler/lib/src/cps_ir/backward_null_check_remover.dart b/pkg/compiler/lib/src/cps_ir/backward_null_check_remover.dart
index 9daa9acf98c57a14e360a5f827b5de955fcc2ff3..422aaa3102216e2b1b64722f8b965714e1a700b7 100644
--- a/pkg/compiler/lib/src/cps_ir/backward_null_check_remover.dart
+++ b/pkg/compiler/lib/src/cps_ir/backward_null_check_remover.dart
@@ -62,6 +62,9 @@ class BackwardNullCheckRemover extends BlockVisitor implements Pass {
     if (prim is InvokeMethod && !selectorsOnNull.contains(prim.selector)) {
       return prim.dartReceiverReference;
     }
+    if (prim is ForeignCode) {
+      return prim.isNullGuardOnNullFirstArgument() ? prim.arguments[0] : null;
+    }
     return null;
   }