constructor() {
      this._lastBundle = {
        pre: new Map(),
        post: new Map(),
        modules: new Map(),
      };
      this._initialized = false;
      this._lastNumModifiedFiles = 0;
      this._lastModifiedDate = new Date();
    }
This is a reference client for the Delta Bundler: it maintains cached the
last patched bundle delta and it's capable of applying new Deltas received
from the Bundler.