Optimize length access on all JSIndexable things.

tests/compiler/dart2js/mock_compiler.dart

Index: tests/compiler/dart2js/mock_compiler.dart
diff --git a/tests/compiler/dart2js/mock_compiler.dart b/tests/compiler/dart2js/mock_compiler.dart
index bd4c6fef190f129ae3dbee691a389ff4488eb42d..dd1e91137166609057be7445a31df0086649baf3 100644
--- a/tests/compiler/dart2js/mock_compiler.dart
+++ b/tests/compiler/dart2js/mock_compiler.dart
@@ -77,7 +77,9 @@ const String DEFAULT_INTERCEPTORSLIB = r'''
     bool operator==(other) => identical(this, other);
     noSuchMethod(im) { throw im; }
   }
-  abstract class JSIndexable {}
+  abstract class JSIndexable {
+    get length;
+  }
   class JSArray extends Interceptor implements List, JSIndexable {
     var length;
     operator[](index) {}