Assert truthiness to pass type check

These will both pass, since we're measuring the box just before.

extensions/amp-ad/0.1/amp-ad-3p-impl.js

@@ -26,7 +26,7 @@ import {isAdPositionAllowed, getAdContainer,}
     from '../../../src/ad-helper';
 import {adConfig} from '../../../ads/_config';
 import {getLifecycleReporter} from '../../../ads/google/a4a/performance';
-import {user} from '../../../src/log';
+import {user, dev} from '../../../src/log';
 import {getIframe} from '../../../src/3p-frame';
 import {setupA2AListener} from './a2a-listener';
 import {moveLayoutRect} from '../../../src/layout-rect';
@@ -202,7 +202,8 @@ export class AmpAd3PImpl extends AMP.BaseElement {
     if (!this.iframeLayoutBox_) {
       this.measureIframeLayoutBox_();
     }
-    return moveLayoutRect(this.iframeLayoutBox_, box.left, box.top);
+    return moveLayoutRect(dev().assert(this.iframeLayoutBox_),
+        box.left, box.top);
   }
 
   /** @override */

extensions/amp-iframe/0.1/amp-iframe.js

@@ -23,7 +23,7 @@ import {listenFor} from '../../../src/iframe-helper';
 import {parseUrl} from '../../../src/url';
 import {removeElement} from '../../../src/dom';
 import {timerFor} from '../../../src/timer';
-import {user} from '../../../src/log';
+import {user, dev} from '../../../src/log';
 import {utf8EncodeSync} from '../../../src/utils/bytes.js';
 import {urls} from '../../../src/config';
 import {moveLayoutRect} from '../../../src/layout-rect';
@@ -237,7 +237,8 @@ export class AmpIframe extends AMP.BaseElement {
     if (!this.iframeLayoutBox_) {
       this.measureIframeLayoutBox_();
     }
-    return moveLayoutRect(this.iframeLayoutBox_, box.left, box.top);
+    return moveLayoutRect(dev().assert(this.iframeLayoutBox_),
+        box.left, box.top);
   }
 
   /** @override */