anchor.diff:      Value notValue = identical(Value.trueInstance, value)
           ? Value.falseInstance
           : Value.trueInstance;
  -    return new ContinuationConfiguration(continuation, notValue);
  +    return new ValuePassingConfiguration(continuation, notValue);
     }
   }
   
  
positive.code:   /// Next continuation to be applied.
    final Continuation continuation;
  
    ExpressionConfiguration(this.expression, this.environment, this.continuation);
  
    Configuration step(StatementExecuter executer) =>
        executer.eval(expression, this);
  }
  
  /// Represents the configuration for evaluating a list of expressions.
  class ExpressionListConfiguration extends Configuration {
    final List<InterpreterExpression> expressions;
    final Environment environment;
    final Continuation continuation;
  
    ExpressionListConfiguration(
        this.expressions, this.environment, this.continuation);
  
    Configuration step(StatementExecuter executer) =>
        executer.evalList(expressions, environment, continuation);
  
negative.code:   }
  }
  
  class NewInstanceConfiguration extends ExecConfiguration {
    final ExpressionContinuation continuation;
    final ObjectValue newObject;
  
  