This ensures we don't add displayName in the transform if it's already been
defined. This is especially important when in strict mode, where duplicate
properties in an object is an exception.
The bug fixed by this commit prevented the correct parsing of
`var` statements with multiple variables being declared. Instead
of trying to parse a whole 'variable declarations' (a `var`
statement with all its declarations), this visitor now only
parses single 'variable declarators'.
