Fixes #139: Use SVG to draw the checkmark.

[bicknellr]

Go to either demo below and resize the window to see the difference:

before: http://jsbin.com/fepide/1/edit?html,output

after: http://jsbin.com/lisubu/1/edit?html,output

(Fixes #139.)

[abdonrd]

Why not use the icon-icons "check" icon?

[bicknellr]

@abdonrd good idea! I rather not make paper-checkbox depend on iron-icon but it's definitely a better source for the SVG than trying to roll my own.

[abdonrd]

@bicknellr why not use the iron-icon element?

[bicknellr]

@rictic

[bicknellr]

@cdata is it ok if I assign this to you?

[danbeam]

yo, what's the status on this?

[dima-gorskov]

When merge this PR?

[e111077]

This visual change will cause quite a bit of jank between the old and new versions which would break a lot of visual tests.

I'm for SVGification, but on the fence about the recentering (as it's non-standard as is).

Thoughts @notwaldorf @rictic?

[e111077]

Would like to know how this PR fares in our internal tests.

[notwaldorf]

(going to remove myself as a reviewer since I think Elliott has this covered!)

[e111077]

@bicknellr any updates on internal testing?

[bicknellr]

Last I checked, it breaks a bunch of screenshot tests - as expected. I don't remember there being any obviously broken non-screenshot tests but I should run it again.

[e111077]

Waiting for update / resolution on this