Add NoSourceLocationMarker in old source-info engine.

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

 // Copyright (c) 2015, 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 js.source_mapping;
 
 import '../io/code_output.dart' show BufferedCodeOutput, SourceLocations;
 import '../io/source_information.dart'
     show SourceLocation, SourceInformation, SourceInformationStrategy;
 import 'js.dart';
 
 /// [SourceInformationStrategy] that can associate source information with
 /// JavaScript output.
 class JavaScriptSourceInformationStrategy extends SourceInformationStrategy {
   const JavaScriptSourceInformationStrategy();
 
   /// Creates a processor that can associate source information on [Node] with
   /// code offsets in the [sourceMapper].
   SourceInformationProcessor createProcessor(SourceMapper sourceMapper) {
     return const SourceInformationProcessor();
   }
 }
 
 /// An observer of code positions of printed JavaScript [Node]s.
 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) {}
 }
 
 /// An interface for mapping code offsets with [SourceLocation]s for JavaScript
 /// [Node]s.
 abstract class SourceMapper {
   /// Associate [codeOffset] with [sourceLocation] for [node].
   void register(Node node, int codeOffset, SourceLocation sourceLocation);
 }
@@ skipping 13 matching lines @@
 
 /// A processor that associates [SourceInformation] with code position of
 /// JavaScript [Node]s.
 class SourceInformationProcessor extends CodePositionListener {
   const SourceInformationProcessor();
 
   /// Process the source information and code positions for the [node] and all
   /// its children.
   void process(Node node, BufferedCodeOutput code) {}
 }