anchor.diff:      var state = new State.initial()
           .withEnvironment(environment)
           .withConfiguration(configuration);
  -    var bodyConfig =
  -        new StatementConfiguration(constructor.function.body, state);
  +    var bodyConfig = new ExecConfiguration(constructor.function.body, state);
   
       // Initialize fields in immediately enclosing class.
       var cont =
           new InstanceFieldsApplication(constructor, environment, bodyConfig);
       var fieldExpressions = _createInstanceInitializers(constructor);
   
  -    return new ExpressionListConfiguration(
  +    return new EvalListConfiguration(
           fieldExpressions, new Environment.empty(), cont);
     }
   
  
positive.code:     if (node.initializer != null) {
        var cont = new VariableInitializerContinuation(
            node, state.environment, state.statementConfiguration);
        return new ExpressionConfiguration(
            node.initializer, state.environment, cont);
      }
      state.environment.expand(node, Value.nullInstance);
      return state.statementConfiguration;
  
negative.code:   Configuration call(Value v);
  }
  
  /// Represents a continuation that returns the next [ExecConfiguration]
  /// to be executed.
  class ExpressionStatementContinuation extends ExpressionContinuation {
    final ExecConfiguration configuration;
  
    ExpressionStatementContinuation(this.configuration);
  
  