parseStatement() {
    // Allow return, yield and await.
    let fs = this.pushFunctionState_(FUNCTION_STATE_LENIENT);
    let result = this.parseModuleItem_(peekType());
    this.popFunctionState_(fs);
    return result;
  }
Parses a single statement. This statement might be a top level statement
in a Script or a Module as well as any other statement allowed in a
FunctionBody.
@return {ParseTree}