static isSnapshotValid(snapshot, mainCompilation) {
    return new Promise((resolve, reject) => {
      mainCompilation.fileSystemInfo.checkSnapshotValid(
        snapshot,
        (err, isValid) => {
          if (err) {
            reject(err);
          }
          resolve(isValid);
        },
      );
    });
  }
Returns true if the files inside this snapshot
have not been changed

@param {Snapshot} snapshot
@param {Compilation} mainCompilation
@returns {Promise<boolean | undefined>}