String toDataUrl([String type = 'image/png', num quality ])

Returns a data URI containing a representation of the image in the format specified by type (defaults to 'image/png').

Data Uri format is as follow data:[<MIME-type>][;charset=<encoding>][;base64],<data>

Optional parameter quality in the range of 0.0 and 1.0 can be used when requesting type 'image/jpeg' or 'image/webp'. If quality is not passed the default value is used. Note: the default value varies by browser.

If the height or width of this canvas element is 0, then 'data:' is returned, representing no data.

If the type requested is not 'image/png', and the returned value is 'data:image/png', then the requested type is not supported.

Example usage:

CanvasElement canvas = new CanvasElement();
var ctx = canvas.context2D
..fillStyle = "rgb(200,0,0)"
..fillRect(10, 10, 55, 50);
var dataUrl = canvas.toDataUrl("image/jpeg", 0.95);
// The Data Uri would look similar to
// 'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUA
// AAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO
// 9TXL0Y4OHwAAAABJRU5ErkJggg=='
//Create a new image element from the data URI.
var img = new ImageElement();
img.src = dataUrl;
document.body.children.add(img);

See also:

Source 

String toDataUrl([String type = 'image/png', num quality]) =>
    _toDataUrl(type, quality);