code: private ParseTree parseExportSpecifier() {
      SourcePosition start = getTreeStartLocation();
      IdentifierToken importedName = eatIdOrKeywordAsId();
      IdentifierToken destinationName = null;
      if (peekPredefinedString(PredefinedName.AS)) {
        eatPredefinedString(PredefinedName.AS);
        destinationName = eatIdOrKeywordAsId();
      }
      return new ExportSpecifierTree(
          getTreeLocation(start), importedName, destinationName);
    }
docs: ExportSpecifier ::= Identifier ('as' Identifier)?