new warnings from the analysis code about final fields

[devoncarew]

var el = new html.ImageElement();
el.src = "image.jpg";

Results in "Field 'src' is final". I am seeing this behavior with other objects as well.

html.CanvasElement canvas = html.document.query('#canvas');
html.CanvasRenderingContext2D ctx = canvas.getContext('2d');
ctx.fillStyle = game.bgStyle;

fillStyle and others give warnings about fields being final.

void onRequest(HttpRequest req, HttpResponse res){
  res.statusCode = HttpStatus.NOT_FOUND;
  res.contentLength = _notFoundPage.length;

statusCode and contentLength do the same.

[scheglov]

Added Duplicate label.
Marked as being merged into #3248.