`_isSymbol` confuses me that `isSymbol("[".charCodeAt(0)) returns false`, but "[" is actual a symbol.
 
To be clear, please change the name `_isSymbol` to `_isPinyin` or other name.
And make the keyboard check before calling `_isSymbol()`
```
    // Select the first candidate if needed.
    if ( code === KeyEvent.DOM_VK_RETURN ||
        this._keyboard !== 'zh-Hans-Pinyin' ||
        !this._isSymbol(code) ||
        this._pendingSymbols.length >= this._kBufferLenLimit) {
    ...
```