fix error when md-button is disabled anchor

[nikita-yaroshevich]

attr.ngDisabled can't be trackable by $watch because it is not inside the scope.

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[gkalpak]

Do you have a testcase that works with this patch and fails without ?

I believe this patch is doing the wrong thing.
ngDisabled holds an Angular expression. The expression itself won't change; it's the expression's evaluated outcome that we want to watch.

[gkalpak]

@ThomasBurleson: Are you sure this patch should have been merged ?
ngDisabled is supposed to hold an expression (docs). It is this expression that needs to be evaluated and watched and not attrs.ngDisabled (which will always return the expression string).

For an example of what I mean, take a look at this demo.

[ThomasBurleson]

@gkalpak - thank you for the recheck. Somehow I merged this without testing myself. I will resolve today.