Add stacktrace_test

tests/compiler/dart2js/sourcemaps/trace_graph.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 sourcemap.trace_graph;
 
 import 'dart:collection';
 
 import 'package:compiler/src/io/source_information.dart';
 import 'package:compiler/src/io/position_information.dart';
 import 'package:compiler/src/js/js_debug.dart';
 
 import 'sourcemap_html_helper.dart';
 
 class TraceGraph {
   List<TraceStep> steps = <TraceStep>[];
   TraceStep entry;
   Queue stack = new Queue();
   Map<int, TraceStep> offsetMap = {};
 
   void addStep(TraceStep step) {
     steps.add(step);
-    int offset = step.offset.subexpressionOffset;
+    int offset = step.offset.value;
     TraceStep existingStep = offsetMap[offset];
     if (existingStep != null) {
       // TODO(johnniwinther): Fix problems with reuse of JS nodes from
       // templates.
       if (identical(existingStep.node, step.node)) {
         print('duplicate node: ${nodeToString(step.node)}');
       } else {
         print('duplicate offset: ${offset} : ${nodeToString(step.node)}');
       }
       print('  ${existingStep.id}:${existingStep.text}:${existingStep.offset}');
@@ skipping 23 matching lines @@
   TraceStep next;
   Map<dynamic, TraceStep> branchMap;
 
   List stack;
 
   TraceStep(this.kind, this.id, this.node, this.offset, this.text,
       [this.sourceLocation]);
 
   String toString() => '<span style="background:${toColorCss(id)}">$id</span>';
 }