anchor.diff:      if (hasExtraArgument) _continuationEnvironment.extend(null, null);
     }
   
  +  /// Construct a collector for collecting only return jumps.
  +  ///
  +  /// There is no jump target, it is implicitly the exit from the function.
  +  /// There is no environment at the destination.
  +  JumpCollector.retrn(this._continuation)
  +      : _continuationEnvironment = null, target = null;
  +
     /// True if the collector has not recorded any jumps to its continuation.
     bool get isEmpty;
   
  
positive.code:   /// A null value indicates that the target is the function's return
    /// continuation.  Otherwise, when inside the try block of try/finally
    /// a return is intercepted to give a place to generate the finally code.
    JumpCollector returnCollector = null;
  
    /// Parameter holding the internal value of 'this' passed to the function.
    ///
  
negative.code:   /// the environment.
    final Map<Local, ClosureLocation> boxedVariables = {};
  
    IrBuilderSharedState(this.program, this.constants, this.currentElement) {
      returnCollector = new ReturnJumpCollector(returnContinuation);
    }
  }
  
  class ThisParameterLocal implements Local {
  