Implement CompilationUnitElement.lineInfo in MockCompilationUnitElement.

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 e9ed649e4cf09548dcfe6c1d7cc520e5dab4f5bb..b56deac697bda4da0f199a8d37ccb63316bc3a19 100644
--- a/pkg/front_end/lib/src/fasta/analyzer/mock_element.dart
+++ b/pkg/front_end/lib/src/fasta/analyzer/mock_element.dart
@@ -13,6 +13,7 @@ import '../builder/builder.dart' show
 
 import '../errors.dart' show
     internalError;
+import 'package:analyzer/src/generated/source.dart';
 
 abstract class MockElement extends Builder
     implements Element, LocalElement {
@@ -211,6 +212,9 @@ abstract class MockCompilationUnitElement extends MockElement
 
   bool get hasLoadLibraryFunction => internalError("not supported.");
 
+  @override
+  LineInfo get lineInfo => internalError("not supported.");
+
   List<TopLevelVariableElement> get topLevelVariables {
     return internalError("not supported.");
   }