message: Expose the requestId fully in the toWidget postMessage API
  This field is flagged as required in the proposal.
  Addresses part of
diff: @@ -51,11 +51,11 @@ export default class ToWidgetPostMessageApi {
  if (payload.response === undefined) {
  return;
  }
  - const promise = this._requestMap[payload._id];
  + const promise = this._requestMap[payload.requestId];
  if (!promise) {
  return;
  }
  - delete this._requestMap[payload._id];
  + delete this._requestMap[payload.requestId];
  promise.resolve(payload);
  }
  @@ -64,21 +64,21 @@ export default class ToWidgetPostMessageApi {
  targetWindow = targetWindow || window.parent; // default to parent window
  targetOrigin = targetOrigin || "*";
  this._counter += 1;
  - action._id = Date.now() + "-" + Math.random().toString(36) + "-" + this._counter;
  + action.requestId = Date.now() + "-" + Math.random().toString(36) + "-" + this._counter;
  return new Promise((resolve, reject) => {
  - this._requestMap[action._id] = {resolve, reject};
  + this._requestMap[action.requestId] = {resolve, reject};
  targetWindow.postMessage(action, targetOrigin);
  if (this._timeoutMs > 0) {
  setTimeout(() => {
  - if (!this._requestMap[action._id]) {
  + if (!this._requestMap[action.requestId]) {
  return;
  }
  console.error("postMessage request timed out. Sent object: " + JSON.stringify(action),
  this._requestMap);
  - this._requestMap[action._id].reject(new Error("Timed out"));
  - delete this._requestMap[action._id];
  + this._requestMap[action.requestId].reject(new Error("Timed out"));
  + delete this._requestMap[action.requestId];
  }, this._timeoutMs);
  }
  });
  