hasExtension method
Returns true if a value of extension is present.
Implementation
bool hasExtension(Extension extension) =>
    _fieldSet._hasExtensions &&
    _fieldSet._extensions!._getFieldOrNull(extension) != null;Returns true if a value of extension is present.
bool hasExtension(Extension extension) =>
    _fieldSet._hasExtensions &&
    _fieldSet._extensions!._getFieldOrNull(extension) != null;