Refactored treatment of closure variables in dart2js CPS.

pkg/compiler/lib/src/tree_ir/tree_ir_tracer.dart

 // Copyright (c) 2014, 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 tree_ir_tracer;
 
 import 'dart:async' show EventSink;
 import '../tracer.dart';
 import 'tree_ir_nodes.dart';
 
@@ skipping 33 matching lines @@
   void _addGotoStatement(Block target) {
     blocks.last.statements.add(target);
   }
 
   void _addBlock(Block block) {
     block.index = blocks.length;
     blocks.add(block);
   }
 
   void collect(ExecutableDefinition node) {
-    visitStatement(node.body);
+    if (node.body != null) {
+      visitStatement(node.body);
+    }
   }
 
   visitLabeledStatement(LabeledStatement node) {
     Block target = new Block(node.label);
     breakTargets[node.label] = target;
     visitStatement(node.body);
     _addBlock(target);
     visitStatement(node.next);
   }
 
@@ skipping 346 matching lines @@
       String prefix = v.element == null ? 'v' : '${v.element.name}_';
       while (name == null || _usedNames.contains(name)) {
         name = "$prefix${_counter++}";
       }
       _names[v] = name;
       _usedNames.add(name);
     }
     return name;
   }
 }