fix a this reference

`this` didn't refer to anything specific, and it was in fact `undefined` in the context it was invoked. There was already a  `let typeTable = this;` further up (where `this` refers to the class instance), so I used `typeTable`.

javascript/extractor/lib/typescript/src/type_table.ts

@@ -1241,7 +1241,7 @@ export class TypeTable {
       stack.push(id);
 
       for (let symbol of type.getProperties()) {
-        let propertyType = this.tryGetTypeOfSymbol(symbol);
+        let propertyType = typeTable.tryGetTypeOfSymbol(symbol);
         if (propertyType == null) continue;
         traverseType(propertyType);
       }