First, I think GWT and the GWT team are awesome. They've created a great piece of technology that I've seen do some amazing things, especially with code sharing.
But... I think Dart is just a much nicer language than Java. It's way more concise and quicker to prototype with due to dynamic typing. Dart also supports reflection, even though it's strongly discouraged for production it's great for getting starting on something very dynamic before writing Dart's equivalent of GWTs generators.
Here's a few things I like better off the top of my head:
- Types are optional
- Fields and methods are in the same namespace, making tearoffs clean.
- Getters and setters. No more getFoo() / setFoo()
- Top level functions
- Implicit interfaces. Every class also defines an interface. No more Foo / FooImpl just because someone _might_ want to implement, or just for tests.
- Mixins
- Constructor initialization is fixed
- Named constructors
- Named arguments
- noSuchMethod() and Function.apply()
- Lambdas and structurally checked function types (not every Java deployment has this yet)
- No shared memory concurrency
- Method cascades.
- Better and more consistent core libraries. Fluent Iterable and Stream used everywhere.
- Factory constructors
- No more builder pattern (named arguments, factories, and cascades combine to make it not needed)
- String interpolation
- Operator overloading, including [] and ==. List and Map use [] and []=, not .get(), etc,. There's no .equals() you just override ==.
- Source-code VM. Edit / refresh, and soon fix-and-continue hot swapping in dart2js.
It adds up. If you look at a reasonably sized ported code sample, it _really_ adds up. It's not fun at all to go back to Java.