title: ng-show binding to string expression in angular
body: <p>Is there a way to bind a ng-show expression to a string which contains the bind expression itself?</p>
  
  <p>For Instance:</p>
  
  <pre><code>field={};
  field.a=true;
  field.showExpression='a == true';
  
  &lt;input ng-show="field.showExpression"&gt;
  </code></pre>
  
  <p>I´ve tried <code>&lt;input ng-show="{{field.showExpression}}"&gt;</code> as well, but none of them seems to work.</p>
  
  <p>I want the bind to stay active, so that when the field.a object changes from true to false the expression gets evaluated again, hiding the input.</p>
  
  <p>Just as background, i´m trying to implement dependant dropdowns, so my showExpressions should be of form field.showExpression='maindropdownValue!=null', and whenever the maindropdown which will be bound to the maindropdownValue gets selected the second one gets displayed.</p>
  
  <p>I´m using angular 1.0.8</p>