title: Dart - cannot get firstWhere to throw exception on no match
body: <p>I am sure this is a simple syntax issue but...</p>
  
  <p>Why does this test throw an exception?
  I expected it to only throw an exception when it could not find any matches.</p>
  
  <pre><code>solo_test( "", (){
    List&lt;String&gt; lines = ["aa=1", "bb=2" ,"cc=3"];
  
    String line = lines.firstWhere((String line) =&gt; line.indexOf("bb") == 0, orElse: throw "could not find 'bb'");
    expect( line, equals( "bb=2"));
  });
  </code></pre>