privateKeyFromMnemonic method
私钥:
Implementation
String privateKeyFromMnemonic(String mnemonic) {
var seed = bip39.mnemonicToSeed(mnemonic);
var root = bip32.BIP32.fromSeed(seed);
var child = root.derivePath("m/44'/60'/0'/0/0");
return bytesToHex(child.privateKey!);
}