Improve precision of JS printer callbacks (2nd try)

pkg/compiler/lib/src/js/js.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;
 
 import 'package:js_ast/js_ast.dart';
 export 'package:js_ast/js_ast.dart';
 
 import '../io/code_output.dart' show CodeBuffer;
@@ skipping 15 matching lines @@
       new Dart2JSJavaScriptPrintingContext(compiler, monitor);
   Printer printer = new Printer(options, context);
   printer.visit(node);
   return context.outBuffer;
 }
 
 class Dart2JSJavaScriptPrintingContext implements JavaScriptPrintingContext {
   final leg.Compiler compiler;
   final DumpInfoTask monitor;
   final CodeBuffer outBuffer = new CodeBuffer();
+  Node rootNode;
 
   Dart2JSJavaScriptPrintingContext(leg.Compiler this.compiler,
       DumpInfoTask this.monitor);
 
   void error(String message) {
     compiler.internalError(NO_LOCATION_SPANNABLE, message);
   }
 
   void emit(String string) {
     outBuffer.add(string);
   }
 
-  void enterNode(Node node) {
+  void enterNode(Node node, int start) {
     SourceInformation sourceInformation = node.sourceInformation;
     if (sourceInformation != null) {
-      sourceInformation.beginMapping(outBuffer);
+      if (rootNode == null) {
+        rootNode = node;
+      }
+      if (sourceInformation.startPosition != null) {
+        outBuffer.addSourceLocation(start, sourceInformation.startPosition);
+      }
     }
-    if (monitor != null) monitor.enteringAst(node, outBuffer.length);
   }
 
-  void exitNode(Node node) {
-    if (monitor != null) monitor.exitingAst(node, outBuffer.length);
+  void exitNode(Node node, int start, int end, int delimiter) {
     SourceInformation sourceInformation = node.sourceInformation;
     if (sourceInformation != null) {
-      sourceInformation.endMapping(outBuffer);
+      if (sourceInformation.endPosition != null) {
+        outBuffer.addSourceLocation(end, sourceInformation.endPosition);
+      }
+      if (rootNode == node) {
+        outBuffer.addSourceLocation(end, null);
+        rootNode = null;
+      }
+    }
+    if (monitor != null) {
+      monitor.recordAstSize(node, end - start);
     }
   }
 }