symbolDefinition method
The symbol key, a full versioned key (kit.kind.variant@N), as a
definition of the current document: the one it holds, or one to add.
Null when no loaded library has key, or while the libraries load or
after they failed (symbolsState tells which).
A ref with an add took its handles from the document's seed, one
per definition and per leaf: call this once per placement, never per
preview. A second call for key before that add executes returns
the same ref, add included (spec 2b S2): execute it once.
Implementation
@override
SymbolDefinitionRef? symbolDefinition(String key) => null;