title: Combine/merge multiple maps into 1 map
body: <p>How can I combine/merge 2 or more maps in dart into 1 map?
  for example I have something like:</p>
  
  <pre><code> var firstMap = {"1":"2"};
   var secondMap = {"1":"2"};
   var thirdMap = {"1":"2"};
  </code></pre>
  
  <p>I want:</p>
  
  <pre><code> var finalMap = {"1":"2", "1":"2", "1":"2"};
  </code></pre>