Allow toggling classes via a boolean value

[DartBot]

This issue was originally filed by @seaneagan

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Often one has a dynamic bool which specifies whether an element should have a given class. For example, a widget's isActive property could map to an 'active' css class. This use case could be supported by adding an optional argument to CssClassSet.toggle as follows:

void toggle(String clazz, [bool contain]) {
  if(contain == null) contain = !contains(clazz);
  contain ? (add : remove)(clazz);
}

I suppose it should probably be added to toggleAll as well, or maybe just remove toggleAll, since one can do:

classes.forEach(el.toggle);

or:

classes.forEach((clazz) => el.toggle(clazz, contain);

[iposva-google]

Added Area-HTML, Triaged labels.

[blois]

Agree that this is more useful than the current toggle.

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cc @efortuna.
Added this to the M6 milestone.
Added jquery label.

[efortuna]

Set owner to @efortuna.

[efortuna]

fixed in r24938

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Added Fixed label.

[DartBot]

This comment was originally written by @sethladd

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Thanks Emily, this is cool!

[DartBot]

This comment was originally written by @sethladd

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For those following along at home, with this patch you can now do:

query('#id').classes.toggle('important', true);

don't forget, you can also do:

query('#id').classes.toggle('important'); which adds if it's not there, and removes if it is there.