title: Constructor syntactic sugar for setting fields in Dart fails when fields are added via mixin
body: <p>Why doesn't constructor syntactic sugar for setting fields work when importing fields from a mixin?</p>
  <pre><code>mixin Nameable { 
  
    var name = '';
  
  }
  
  class Person with Nameable {
  
    Person(this.name);
  
  }
  </code></pre>
  <p>Fails with error-message <code>'name' isn't a field in the enclosing class.</code></p>
  <p>This is ok though</p>
  <pre><code>mixin Nameable { 
  
    var name = '';
  
  }
  
  class Person with Nameable {
  
    Person(String name) {
      this.name = name;
    }
  
  }
  </code></pre>
  <p>Report as a bug?</p>