Optimize length access on all JSIndexable things.

tests/utils/recursive_import_test.dart

Index: tests/utils/recursive_import_test.dart
diff --git a/tests/utils/recursive_import_test.dart b/tests/utils/recursive_import_test.dart
index b6e95f301a4c9b7dca2173295cbfe2fd7dd63185..a9351c133e8f1dfa535f1a08026dd93c14d54ff8 100644
--- a/tests/utils/recursive_import_test.dart
+++ b/tests/utils/recursive_import_test.dart
@@ -36,13 +36,14 @@ class JSInvocationMirror {}  // Should be in helper.
 
 const INTERCEPTORS_LIB = """
 library interceptors;
+class JSIndexable {
+  get length;
+}
 class JSArray {
-  get length => null;
   removeLast() => null;
   add(x) { }
 }
 class JSString {
-  get length => null;
   split(x) => null;
   concat(x) => null;
   toString() => null;