Some people will see this as ideal. I've known many smart and capable programmers who hate type systems. What I see as a helpful exoskeleton of logic, they see as a rigid pain in the neck invented by programmers who want to get paid by the keystroke. Dart offers them most of the power of type checking if and when they want to deploy it. If they're just having fun and writing something simple, there's no need to be too verbose and specify the type. People like me, on the other hand, can add types to everything, and it might help catch some bugs. This kind of flexibility is ideal for the lone coder because the lone coder can choose their favorite path.
A deeper question is whether Dart and this new freedom will really make it easier to get teams together to write big programs. There's no doubt that the careful namespaces and class structure add nice firewalls that prevent collisions. That discipline is something that's bound to help most people.
But can all of the other new features and flexibility help larger teams? In some cases, the syntactical shorthand may sow confusion. In others, the devil-may-care attitude toward typing will leave team members at odds with each other because flexibility isn't always a good idea for keeping people on the same path.
Google Dart: Structure and freedom I've worked on several teams where the leader was a real whip cracker who drafted long lists of annoying rules. Each statement had to be on its own line. Each constant had to start with a certain letter. There could be no chaining of method calls because it made stepping through the code difficult during debugging. Everyone hated the rules and the rule maker, but the code was much more consistent and readable.
Dart seems to be taking the opposite approach. There are often several ways to say exactly the same thing. Functions are first-class objects and they're tossed everywhere. Constructors take various forms. Of course you can also get your own whip cracker to come up with tough rules and jam them down the throats of the coders, but we can already do that with the existing languages. If anything, Dart is offering more ways for programmers to develop their own secret idioms, then get cheesed off when the other coders can't grok their infinite coolness.