title: in defining a function in Dart, how to set an argument's default to { }, ie, an empty Map?
body: <p>love how <code>Dart</code> treats function arguments, but cannot accomplish what should be a simple task:</p>
  
  <pre>
    void func( String arg1, [ Map args = {} ] ) {
      ...
    }
  </pre>
  
  <p>get the error</p>
  
  <pre>
    expression is not a valid compile-time constant
  </pre>
  
  <p>have tried <code>new Map()</code> for example, with same error.</p>