Google is a web company ( at least in their heads ) and Dart was supposed to replace Javascript ( lol ), they used GWT for their projects since forever and now many use Dart and Angular - Adsense for sure and many other very important products for them ( $$$ ) so it's important they capture the Web side of things with Flutter because Google and their PMs and VPs are the most important clients in order to ensure the future of Flutter.

Please note I didn't mentioned anything technical, it's pure product management and strategy and that's one of the reasons I'm optimistic about Flutter's future.