anchor.diff:      values.remove(node.variable);
     }
   }
  -
  -
  -class ContainsTry extends RecursiveVisitor {
  -  bool _found = false;
  -  ContainsTry._();
  -
  -  /// Scans [root] for evidence of try-catch and try-finally.
  -  static bool analyze(Node root) {
  -    ContainsTry visitor = new ContainsTry._();
  -    visitor.visit(root);
  -    return visitor._found;
  -  }
  -
  -  visit(Node node) {
  -    if (_found) return;  // Early exit if we know the answer.
  -    super.visit(node);
  -  }
  -
  -  processLetHandler(LetHandler node) {
  -    _found = true;
  -  }
  -}
  
positive.code: 
  import 'optimizers.dart';
  
  import '../closure.dart' show
      ClosureClassElement, Identifiers;
  import '../common/names.dart' show
      Selectors, Identifiers;
  import '../compiler.dart' as dart2js show
      Compiler;
  import '../constants/constant_system.dart';
  
negative.code:     return false;
    }
  
    /// Side-effect free expressions with constant results are be replaced by:
    ///
    ///    (LetPrim p = constant (InvokeContinuation k p)).
  