code: private ParseTree parseImportSpecifier() {
      SourcePosition start = getTreeStartLocation();
      IdentifierToken importedName = eatIdOrKeywordAsId();
      IdentifierToken destinationName = null;
      if (peekPredefinedString(PredefinedName.AS)) {
        eatPredefinedString(PredefinedName.AS);
        destinationName = eatId();
      } else if (isKeyword(importedName.value)) {
        reportExpectedError(null, PredefinedName.AS);
      }
      return new ImportSpecifierTree(
          getTreeLocation(start), importedName, destinationName);
    }
docs: ImportSpecifier ::= Identifier ('as' Identifier)?