Fix issue where errors aren't reported for a new overlay

pkg/analyzer/test/generated/engine_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.test.generated.engine_test;
 
 import 'dart:async';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
@@ skipping 559 matching lines @@
   }
 
   @override
   List<Source> getSourcesWithFullName(String path) {
     fail("Unexpected invocation of getSourcesWithFullName");
     return null;
   }
 
   @override
   bool handleContentsChanged(
-      Source source, String originalContents, String newContents, bool notify) {
+      Source source, String originalContents, String newContents, bool notify,
+      {bool explicit: false}) {
     fail("Unexpected invocation of handleContentsChanged");
     return false;
   }
 
   @override
   void invalidateLibraryHints(Source librarySource) {
     fail("Unexpected invocation of invalidateLibraryHints");
   }
 
   @override
@@ skipping 87 matching lines @@
   bool validateCacheConsistency() {
     fail("Unexpected invocation of validateCacheConsistency");
     return false;
   }
 
   @override
   void visitContentCache(ContentCacheVisitor visitor) {
     fail("Unexpected invocation of visitContentCache");
   }
 }