title: setState is not updating value
body: <p><strong>Problem:</strong>
  setState is not saving the value of <em>_products.</em></p>
  
  <p><strong>Tried:</strong>
  If I print the value of <em>_products</em> inside the setState function, it is updated: </p>
  
  <pre><code>['Product 1', 'Product 2', 'New Product']
  </code></pre>
  
  <p>But when I pass <em>_products</em> to my <em>Products</em> class, the value of <em>_products</em> is reset:</p>
  
  <pre><code>['Product 1', 'Product 2']
  </code></pre>
  
  <p><strong>Code:</strong></p>
  
  <pre><code>import 'package:flutter/material.dart';
  
  import './products.dart';
  
  class ProductManager extends StatefulWidget {
    @override
    State&lt;StatefulWidget&gt; createState() {
      return _ProductManagerState();
    }
  }
  
  class _ProductManagerState extends State&lt;ProductManager&gt; {
    @override
    Widget build(BuildContext context) {
  
      List&lt;String&gt; _products = ['Product 1', 'Product 2'];
      return Column(
        children: [
          Container(
            margin: EdgeInsets.all(10.0),
            child: RaisedButton(
              onPressed: () {
                setState(() {
                  _products.add('New Product');
                });
              },
              child: Text('Add Product'),
            ),
          ),
          Products(_products),
        ],
      );
    }
  }
  </code></pre>