withMockControl() {
    if (!this.shouldMakeMockControl_) {
      this.shouldMakeMockControl_ = true;
      this.mockControl = new MockControl();
    }
    return this;
  }
Create a new {@see MockControl} accessible via
`env.mockControl` for each test. If your test has more than one
testing environment, don't call this on more than one of them.
@return {!Environment} For chaining.