anchor.diff:      if (location.endsWith('.dart')) {
         location += '.js';
       } else {
  -      throw new ArgumentError('$location does not point to a .dart file');
  +      return new ArgumentError('$location does not point to a .dart file');
       }
       // See if it already exists
       var allScripts = querySelectorAll('script');
  
positive.code:           return await script.onLoad.first.then((e) {
              script.setAttribute('loaded', 'true');
              return script;
            });
          }
        }
  
negative.code:     return await script.onLoad.first.then((e) {
        script.setAttribute('loaded', 'true');
        return script;
      }).catchError((e) {
        return e;
      });
    } else {
      return spawnDomUri(uri, [], '');
    }
  }