anchor.diff:  
     Configuration visitReturnStatement(ReturnStatement node, State state) {
       assert(state.returnContinuation != null);
  -    // The new ExpressionState contains the next expression continuation.
  -    var expState = new ExpressionState.fromStatementState(state)
  -        .withContinuation(state.returnContinuation);
  +    if (node.expression == null) {
  +      return new ContinuationConfiguration(
  +          state.returnContinuation, Value.nullInstance);
  +    }
  +
       return new ExpressionConfiguration(
  -        node.expression ?? new NullLiteral(), expState);
  +        node.expression, state.environment, state.returnContinuation);
     }
   
     Configuration visitVariableDeclaration(
         VariableDeclaration node, State state) {
       if (node.initializer != null) {
  -      var expState = new ExpressionState.fromStatementState(state);
         var cont = new VariableInitializerContinuation(
             node, state.environment, state.statementConfiguration);
         return new ExpressionConfiguration(
  -          node.initializer, expState.withContinuation(cont));
  +          node.initializer, state.environment, cont);
       }
       state.environment.expand(node, Value.nullInstance);
       return state.statementConfiguration;
  
positive.code:   }
  
    Configuration visitWhileStatement(WhileStatement node, State state) {
      var expState = new ExpressionState.fromStatementState(state);
      var cont = new WhileConditionContinuation(node, state);
  
      return new ExpressionConfiguration(
          node.condition, expState.withContinuation(cont));
    }
  
    Configuration visitDoStatement(DoStatement node, State state) {
  
negative.code:   }
  
    Configuration visitIfStatement(IfStatement node, State state) {
      var cont = new IfConditionContinuation(node.then, node.otherwise, state);
  
      return new ExpressionConfiguration(node.condition, state.environment, cont);
    }
  
    Configuration visitLabeledStatement(LabeledStatement node, State state) {
  