[JS] Fix Parser.getSourceName returning undefined

Parser.getSourceName was expecting a field `sourceName` on `TokenStream`,
but that doesn't exist on any `TokenStream` implementation. This commit
fixes the bug by calling `getSourceName`, which *is* defined on
`BufferedTokenStream`.

This commit also removes some nearby commented-out code that didn't make
any sense.

Signed-off-by: Phlosioneer <mattmdrr2@gmail.com>

runtime/JavaScript/src/antlr4/Parser.js

@@ -603,14 +603,8 @@ export default class Parser extends Recognizer {
         }
     }
 
-    /*
-        "			printer = function() {\r\n" +
-        "				this.println = function(s) { document.getElementById('output') += s + '\\n'; }\r\n" +
-        "				this.print = function(s) { document.getElementById('output') += s; }\r\n" +
-        "			};\r\n" +
-        */
     getSourceName() {
-        return this._input.sourceName;
+        return this._input.getSourceName();
     }
 
     /**