Refactor Canvas 2D context taming to prototype style.

* CanvasRenderingContext2D, CanvasGradient, and ImageData have regular
  taming ctors and prototypes rather than closure-based wrappers.
* Method taming tools used by canvas taming are now implemented as
  property specs as used in the rest of Domado.
* Removed drawFocusRing taming as drawFocusRing no longer exists in
  spec or implementations.
* 2D canvas is no longer fetched overeagerly and <canvas>.getContext is
  capable of supporting other context types (if we had tamings for
  them).

Supporting changes in es53-test-scan-guest:
* Refer to canvas methods via prototype rather than string matching.
* Fix bugs in "Show only errors" checkbox.

@r5422

[kpreid2, 2013-05-23 18:54:17]

* CanvasRenderingContext2D, CanvasGradient, and ImageData have regular
  taming ctors and prototypes rather than closure-based wrappers.
* Method taming tools used by canvas taming are now implemented as
  property specs as used in the rest of Domado.
* Removed drawFocusRing taming as drawFocusRing no longer exists in
  spec or implementations.
* 2D canvas is no longer fetched overeagerly and <canvas>.getContext is
  capable of supporting other context types (if we had tamings for
  them).

Supporting changes in es53-test-scan-guest:
* Refer to canvas methods via prototype rather than string matching.
* Fix bugs in "Show only errors" checkbox.

[felix8a, 2013-05-23 20:46:27]

lgtm++

some of the comments are about code that was just moved, not changed. I started
looking at all that in detail since rietveld doesn't know how to align the
diffs, but halfway through my attention started wandering, so I threw the patch
into diffmerge and focused on just the parts that were changed.

https://codereview.appspot.com/9698043/diff/3001/src/com/google/caja/plugin/d...
File src/com/google/caja/plugin/domado.js (right):

https://codereview.appspot.com/9698043/diff/3001/src/com/google/caja/plugin/d...
src/com/google/caja/plugin/domado.js:4799: // http://dev.w3.org/html5/2dcontext/
I usually look at whatwg instead of w3c, how about list both?
http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-elemen...

also, mdn is often a better guide to what's actually implemented by browsers
(not just by firefox)
https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D

https://codereview.appspot.com/9698043/diff/3001/src/com/google/caja/plugin/d...
src/com/google/caja/plugin/domado.js:4815: return '[Domado
CanvasRenderingContext2D]';
'[domado object ...'

https://codereview.appspot.com/9698043/diff/3001/src/com/google/caja/plugin/d...
src/com/google/caja/plugin/domado.js:4818: save: tameSimpleOp,
'simple' is vague, I'd prefer something like 'tameNoArgOp'

https://codereview.appspot.com/9698043/diff/3001/src/com/google/caja/plugin/d...
src/com/google/caja/plugin/domado.js:4825: setTransform: tameFloatsOp(6),
note unimplemented: whatwg has resetTransform, though nobody seems to implement
it yet

https://codereview.appspot.com/9698043/diff/3001/src/com/google/caja/plugin/d...
src/com/google/caja/plugin/domado.js:4829: enforceType(x0, 'number', 'x0');
I think I'd prefer removing most of the enforceType 'number' in the canvas code
and just coercing to number with unary +. mainly, the cost and codesize of all
that redundant checking seems not worth it.

https://codereview.appspot.com/9698043/diff/3001/src/com/google/caja/plugin/d...
src/com/google/caja/plugin/domado.js:4867: arcTo: tameFloatsOp(5),
note unimplemented: whatwg adds two optional args to arcTo

https://codereview.appspot.com/9698043/diff/3001/src/com/google/caja/plugin/d...
src/com/google/caja/plugin/domado.js:4876: enforceType(anticlockwise, 'boolean',
'anticlockwise');
anticlockwise is an optional arg

https://codereview.appspot.com/9698043/diff/3001/src/com/google/caja/plugin/d...
src/com/google/caja/plugin/domado.js:4877: if (radius < 0) {
I don't think we gain anything by doing this check ourselves.

https://codereview.appspot.com/9698043/diff/3001/src/com/google/caja/plugin/d...
src/com/google/caja/plugin/domado.js:4885: fill: tameSimpleOp,
note unimplemented: w3c and whatwg say that fill, stroke, and clip can have a
path as arg 1, and w3c says fill and clip can have a string as arg 1.

https://codereview.appspot.com/9698043/diff/3001/src/com/google/caja/plugin/d...
src/com/google/caja/plugin/domado.js:4891: isPointInPath:
tameMethodCustom(function(privates, x, y) {
note unimplemented: w3c and whatwg have 3 arg variant, and w3c has a 4 arg
variant

https://codereview.appspot.com/9698043/diff/3001/src/com/google/caja/plugin/d...
src/com/google/caja/plugin/domado.js:4906: // Consider what policy to have wrt
reading the pixels from image
mark this with TODO. many canvas things are pretty much not doable without
drawImage

https://codereview.appspot.com/9698043/diff/3001/src/com/google/caja/plugin/d...
src/com/google/caja/plugin/domado.js:4908: throw new Error('Domita: canvas
drawImage not yet implemented');
domado