Add NoSourceLocationMarker in old source-info engine.

pkg/compiler/lib/src/js/js_source_mapping.dart

Index: pkg/compiler/lib/src/js/js_source_mapping.dart
diff --git a/pkg/compiler/lib/src/js/js_source_mapping.dart b/pkg/compiler/lib/src/js/js_source_mapping.dart
index df49903d8a41ec64b3775a9cdd528dd282d4f83e..11278374bf815195faf6b967a368e465e979b0fb 100644
--- a/pkg/compiler/lib/src/js/js_source_mapping.dart
+++ b/pkg/compiler/lib/src/js/js_source_mapping.dart
@@ -25,8 +25,15 @@ class JavaScriptSourceInformationStrategy extends SourceInformationStrategy {
 class CodePositionListener {
   const CodePositionListener();
 
+  /// Called to associate [node] with the provided start position.
+  ///
+  /// The nodes are seen in pre-traversal order.
+  void onStartPosition(Node node, int startPosition) {}
+
   /// Called to associate [node] with the provided start, end and closing
   /// positions.
+  ///
+  /// The nodes are seen in post-traversal order.
   void onPositions(
       Node node, int startPosition, int endPosition, int closingPosition) {}
 }