add source maps support, fixes #50

lib/src/js/nodes.dart

 // Copyright (c) 2012, 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.
 
 part of js_ast;
 
 abstract class NodeVisitor<T> {
   T visitProgram(Program node);
 
   T visitBlock(Block node);
@@ skipping 185 matching lines @@
       => visitInterpolatedNode(node);
 
   // Ignore comments by default.
   T visitComment(Comment node) => null;
   T visitCommentExpression(CommentExpression node) => null;
 
   T visitAwait(Await node) => visitExpression(node);
   T visitDartYield(DartYield node) => visitStatement(node);
 }
 
-/// This tag interface has no behaviour but must be implemented by any class
-/// that is to be stored on a [Node] as source information.
-abstract class JavaScriptNodeSourceInformation {}
-
 abstract class Node {
-  JavaScriptNodeSourceInformation get sourceInformation => _sourceInformation;
-
-  JavaScriptNodeSourceInformation _sourceInformation;
+  /// Sets the source location of this node. For performance reasons, we allow
+  /// setting this after construction.
+  Object sourceInformation;
 
   accept(NodeVisitor visitor);
   void visitChildren(NodeVisitor visitor);
 
   // Shallow clone of node.  Does not clone positions since the only use of this
   // private method is create a copy with a new position.
   Node _clone();
 
   // Returns a node equivalent to [this], but with new source position and end
   // source position.
-  Node withSourceInformation(
-      JavaScriptNodeSourceInformation sourceInformation) {
-    if (sourceInformation == _sourceInformation) {
+  Node withSourceInformation(sourceInformation) {
+    if (sourceInformation == this.sourceInformation) {
       return this;
     }
     Node clone = _clone();
     // TODO(sra): Should existing data be 'sticky' if we try to overwrite with
     // `null`?
-    clone._sourceInformation = sourceInformation;
+    clone.sourceInformation = sourceInformation;
     return clone;
   }
 
   VariableUse asVariableUse() => null;
 
   bool get isCommaOperator => false;
 
   Statement toStatement() {
     throw new UnsupportedError('toStatement');
   }
@@ skipping 1166 matching lines @@
   final Expression expression;
 
   CommentExpression(this.comment, this.expression);
 
   int get precedenceLevel => PRIMARY;
   accept(NodeVisitor visitor) => visitor.visitCommentExpression(this);
   CommentExpression _clone() => new CommentExpression(comment, expression);
 
   void visitChildren(NodeVisitor visitor) => expression.accept(visitor);
 }