fix initialization of null in loops

frog/gen.dart

 // Copyright (c) 2011, 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.
 
 /**
  * Top level generator object for writing code and keeping track of
  * dependencies.
  *
  * Should have two compilation models, but only one implemented so far.
  *
@@ skipping 1069 matching lines @@
           world.error('no value specified for final variable', node.span);
         } else {
           // TODO(jimhug): Mark inferred types as special for correct errors.
           if (thisType.isVar) thisType = value.type;
         }
       }
 
       var val = _scope.create(name, thisType, node.names[i].span);
 
       if (value == null) {
-        writer.write('${val.code}');
+        if (_scope.reentrant) {
+          // To preserve block scoping, we need to ensure the variable is
+          // reinitialized each time the block is entered.
+          writer.write('${val.code} = null');
+        } else {
+          writer.write('${val.code}');
+        }
       } else {
         value = value.convertTo(this, type, node.values[i]);
         writer.write('${val.code} = ${value.code}');
       }
     }
     writer.writeln(';');
     return false;
 
   }
 
@@ skipping 1145 matching lines @@
       result.add(new Value(world.varType, '\$$i', null, /*needsTemp:*/false));
     }
     for (int i = bareCount; i < length; i++) {
       var name = getName(i);
       if (name == null) name = '\$$i';
       result.add(new Value(world.varType, name, null, /*needsTemp:*/false));
     }
     return new Arguments(nodes, result);
   }
 }