title: Dart: Map class, reduce (or otherwise find certain information)
body: <p>New to Dart (Flutter), and the docs don't seem to have a method for the Map class that will allow me to do this easily...</p>
  
  <p>I want to have a Map with keys of Datetime and values of calories eaten. </p>
  
  <pre><code>{'2019-07xxx': 350, '2019-07xxx': 500, ...}
  </code></pre>
  
  <p>Now, what's the best way to filter this so that I only get values from today? (i.e. when starting the app and pulling the data from storage)</p>
  
  <p>Ideally once I do that, I can get the cumulative value of today's calories so far with:</p>
  
  <pre><code>var sum = todaysCaloriesArray.reduce((a, b) =&gt; a + b);
  </code></pre>
  
  <p>Unless there is some Dart-fu that would allow this in a better way? </p>