Copy all fields of NullCheck

pkg/compiler/lib/src/js_backend/codegen/codegen.dart

Index: pkg/compiler/lib/src/js_backend/codegen/codegen.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
index 8f3819f465407ec06aa6dc9a2cfcb502cb0dbc70..7fff9a15a8138f331fa44a886f30cfe42fa1f06e 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
@@ -1021,7 +1021,15 @@ class CodeGenerator extends tree_ir.StatementVisitor
   @override
   void visitNullCheck(tree_ir.NullCheck node) {
     js.Expression value = visitExpression(node.value);
-    js.Expression access = node.selector != null
+    // TODO(sra): Try to use the selector even when [useSelector] is false. The
+    // reason we use 'toString' is that it is always defined so avoids a slow
+    // lookup (in V8) of an absent property. We could use the property for the
+    // selector if we knew it was present. The property is present if the
+    // associated method was not inlined away, or if there is a noSuchMethod
+    // hook for that selector. We don't know these things here, but the decision
+    // could be deferred by creating a deferred property that was resolved after
+    // codegen.
+    js.Expression access = node.selector != null && node.useSelector
         ? js.js('#.#', [value, glue.invocationName(node.selector)])
         : js.js('#.toString', [value]);
     if (node.condition != null) {