anchor.diff:  
     Configuration call(Value receiver) {
       var cont = new SetterContinuation(receiver, setterName, continuation);
  -    return new ExpressionConfiguration(value, environment, cont);
  +    return new EvalConfiguration(value, environment, cont);
     }
   }
   
  
positive.code:     for (InterpreterValue v in values.reversed) {
        result.write(v.value.value);
      }
      return new ContinuationConfiguration(
          continuation, new StringValue(result.toString()));
    }
  }
  
negative.code:   Configuration call(Value receiver) {
      // TODO: CPS the invocation of the getter.
      Value propertyValue = receiver.class_.lookupGetter(name)(receiver);
      return new ValuePassingConfiguration(continuation, propertyValue);
    }
  }
  
  