Optimize length access on all JSIndexable things.

tests/utils/dummy_compiler_test.dart

Index: tests/utils/dummy_compiler_test.dart
diff --git a/tests/utils/dummy_compiler_test.dart b/tests/utils/dummy_compiler_test.dart
index 7f145cb0a619d3db5f6ff38dcdff4a3cee59163f..c488b9768823b3b26af98a58a21f0de95d7f75e9 100644
--- a/tests/utils/dummy_compiler_test.dart
+++ b/tests/utils/dummy_compiler_test.dart
@@ -43,15 +43,16 @@ Future<String> provider(Uri uri) {
       source = '';
     } else if (uri.path.endsWith('interceptors.dart')) {
       source = """class ObjectInterceptor {}
+                  class JSIndexable {
+                    get length;
+                  }
                   class JSArray {
-                    var length;
                     var removeLast;
                     var add;
                   }
                   class JSFixedArray {}
                   class JSExtendableArray {}
                   class JSString {
-                    var length;
                     var split;
                     var concat;
                     var toString;