Ensure locations are always provided, but don't store them yet.

pkg/front_end/lib/src/fasta/analyzer/mock_element.dart

Index: pkg/front_end/lib/src/fasta/analyzer/mock_element.dart
diff --git a/pkg/front_end/lib/src/fasta/analyzer/mock_element.dart b/pkg/front_end/lib/src/fasta/analyzer/mock_element.dart
index a55197bff6615dd26ea82cad57a9d4caf8ced2ce..f5875bade38ea9adcafb315ec8ec16b091304955 100644
--- a/pkg/front_end/lib/src/fasta/analyzer/mock_element.dart
+++ b/pkg/front_end/lib/src/fasta/analyzer/mock_element.dart
@@ -6,13 +6,17 @@ library fasta.analyzer.mock_element;
 
 import 'package:analyzer/dart/element/element.dart';
 
+import '../builder/builder.dart' show
+    Builder;
+
 import '../errors.dart' show
     internalError;
 
-abstract class MockElement implements Element {
+abstract class MockElement extends Builder implements Element {
   final ElementKind kind;
 
-  MockElement(this.kind);
+  MockElement(this.kind)
+      : super(null, -1, null);
 
   get librarySource => internalError("not supported.");