This issue was originally filed by @denniskaselow

What steps will reproduce the problem?

1. Run this code as JavaScript
   import 'dart:html';

void main() {
  var canvas = new CanvasElement();
  canvas.context2D.fill(null);
}

What is the expected output? What do you see instead?
Works in Dart but does not work in JS.
When run in JS, following error occurs:
TypeError: Argument 1 of CanvasRenderingContext2D.fill 'null' is not a valid value for enumeration CanvasWindingRule.

What version of the product are you using? On what operating system?
Dart Editor version 1.0.0_r30188 (STABLE)
Dart SDK version 1.0.0.3_r30188

Please provide any additional information below.
I'm not sure if it really is a bug but at least it's a bit of an inconsistency between Dart and JS (error can probably pop up for similar functions). In my case I am delegating a call to fill and just pass the optional parameter without checking for null (easy workaround: I check for null and call the method without a param). Also, no need to bring ?param back ;).