Make constraint unbinding more robust

[Delivers #83698142]
Implemented #83637396. Setters now return the value to be set.
Merge pull request #71 from teem2/setterreturn

Implemented #83637396. Setters will now return the value to be set.
Commented out the assert in smoke/node.html since the behavior it tests is not well defined yet and is not likely to be so anytime soon.
Implemented #83729086. Calling super is now safer. Also streamlined the super implementation to move the conditional out of the generated method.
Merge pull request #73 from teem2/safersuper

Implemented #83729086. Calling super is now safer. Also streamlined the ...
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