Start using the new union/intersection code for HType.INDEXABLE_PRIMITIVE.

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 149ccf3cbd5e70839a8873d1ae2cfaabe02b6194..ca8b01e2cf8e6804b7f316e0c218d9eccbcf72f8 100644
--- a/tests/compiler/dart2js/mock_compiler.dart
+++ b/tests/compiler/dart2js/mock_compiler.dart
@@ -63,13 +63,14 @@ const String DEFAULT_HELPERLIB = r'''
   throwAbstractClassInstantiationError(className) {}''';
 
 const String DEFAULT_INTERCEPTORSLIB = r'''
-  class JSArray implements List {
+  class JSIndexable {}
+  class JSArray implements List, JSIndexable {
     var length;
     operator[](index) {}
     operator[]=(index, value) {}
     var add;
   }
-  class JSString implements String {
+  class JSString implements String, JSIndexable {
     var length;
     operator[](index) {}
     toString() {}