This PR fixes a bug that caused spurious validation error markings on conditional successions in the Xtext SysML editor in Eclipse.

For example, in the following model, the source of the conditional succession s is implicitly the preceding action a:

action def A {
  action a;
  if true than b;
  action b;
}

Previously, this would initially parse correctly in the Xtext editor without validation errors. But, if it was part of a larger model, then, when a change was made elsewhere in the model, after incremental re-parsing, the source on the conditional succession was not set correctly, resulting in the validation error “Must have at least two related elements”.

A so-called "conditional succession" is actually parsed as a TransitionUsage, and an implied source such as above is computed in the TransitionUsage::computeSource method. This PR updates the computeSource method to handle the case when transformations are re-run on a conditional succession in the abstract syntax tree, when it has not actually been re-parsed.