Type tracing bug fix for issue 28919.

tests/compiler/dart2js/mock_libraries.dart

Index: tests/compiler/dart2js/mock_libraries.dart
diff --git a/tests/compiler/dart2js/mock_libraries.dart b/tests/compiler/dart2js/mock_libraries.dart
index 14cb27809c2ad6682c65dacb7c8ec20519c10882..0c10523de66379564033a5b09fa47a3b0aa82862 100644
--- a/tests/compiler/dart2js/mock_libraries.dart
+++ b/tests/compiler/dart2js/mock_libraries.dart
@@ -304,7 +304,7 @@ const Map<String, String> DEFAULT_INTERCEPTORS_LIBRARY = const <String, String>{
             var length;
             operator[](index) => this[index];
             operator[]=(index, value) { this[index] = value; }
-            add(value) { this[length + 1] = value; }
+            add(value) { this[length] = value; }
             insert(index, value) {}
             E get first => this[0];
             E get last => this[0];
@@ -313,7 +313,7 @@ const Map<String, String> DEFAULT_INTERCEPTORS_LIBRARY = const <String, String>{
             E removeAt(index) => this[0];
             E elementAt(index) => this[0];
             E singleWhere(f) => this[0];
-            Iterator<E> get iterator => null; 
+            Iterator<E> get iterator => null;
           }''',
   'JSBool': 'class JSBool extends Interceptor implements bool {}',
   'JSDouble': 'class JSDouble extends JSNumber implements double {}',