An end usage is automatically considered to be constant if it may time vary. However, while isConstant() returned true for such elements in memory, the value isConstant = true was not being serialized to XMI for them. Similarly, the isAbstract property was not being serialized for variation definitions and usages, even though isAbstract() returns true for them.

The underlying problem is that the automatic setting of isConstant and isAbstract was bing done by overriding the methods isConstant() in UsageImpl and isAbstract in DefinitionImpl and UsageImpl. In both case, the corresponding the field actually retained its default value. During XMI serialization, the eIsSet operation is used to check if a field is set. Unfortunately, the generated implementation for this for both isConstant and isAbstract directly checks the field against the default value. These fields are thus treated as having the default value and not serialized.

This PR fixes the problem as follows.

1. The "additional override" for isConstant() has been removed from UsageImpl. Instead, in UsageAdapter::transform, the isConstant property is explicitly set to true if the usage is an end and it may time vary. (Note that this cannot be done in postProcess, because the computation of mayTimeVary requires traversing the specializations of the usage, which cannot be properly done during parse post-processing.
2. The "additional overrides" for isAbstract() have been removed from DefinitionImpl and UsageImpl. Instead DefinitionAdapter::postProcess and UsageAdapter::postProcess have been updated to explicitly set isAbstract to true if isVariation is true.