Model Builder: Fix ordering issue #3

sketchtool dump includes a <class> param that allows Sketch to generate a pretty accurate model in the SketchTypeBuilder class. It is able to infer optionality if a key appears in a given JSON object and doesn't appear in later JSON object.

However, the implementation is not pretty, and it currently doesn't work if the key is not present in the first JSON object and then is present in the second JSON object.

This bug can be worked around when encountered by re-ordering things in the InputSketch file, but it would be far more trustworthy if it worked correctly.