title: Add leading zeroes to number in Dart
body: <p>I want to add some leading zeroes to a string. For example, the total length may be eight characters. For example:</p>
  
  <pre><code>123 should be 00000123
  1243 should be 00001234
  123456 should be 00123456
  12345678 should be 12345678
  </code></pre>
  
  <p>What is an easy way to do this in Dart?</p>