Base JavaScript code position computation on JavaScript tracer.

pkg/js_ast/lib/src/nodes.dart

Index: pkg/js_ast/lib/src/nodes.dart
diff --git a/pkg/js_ast/lib/src/nodes.dart b/pkg/js_ast/lib/src/nodes.dart
index b6d11aa9988096635af8327233c25e91f0540cf7..07a20f1312ad11d33d8fa9b83dd802ebcbd846c4 100644
--- a/pkg/js_ast/lib/src/nodes.dart
+++ b/pkg/js_ast/lib/src/nodes.dart
@@ -195,7 +195,9 @@ class BaseVisitor<T> implements NodeVisitor<T> {
 
 /// 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 JavaScriptNodeSourceInformation {
+  const JavaScriptNodeSourceInformation();
+}
 
 abstract class Node {
   JavaScriptNodeSourceInformation get sourceInformation => _sourceInformation;
@@ -1075,7 +1077,7 @@ class LiteralNumber extends Literal {
   LiteralNumber(this.value);
 
   int get precedenceLevel => value.startsWith('-') ? UNARY : PRIMARY;
-  
+
   accept(NodeVisitor visitor) => visitor.visitLiteralNumber(this);
   LiteralNumber _clone() => new LiteralNumber(value);
 }