>Never heard anyone say Dart's package system is better than NPM

neither do I - just say Dart has good dedicated (npm is for JS AFAIK) package system

>Latency makes Eclipse unusable on every machine I've tried

It's not as good as lets say IntelliJ (best IDE I've ever seen) but it's "good enough"

Besides - it seems long term goal for Dart team is chromedeveditor https://github.com/dart-lang/chromedeveditor (another point for Dart it's open source) which will be pretty fast when DartVM will come to Chrome

Trying Chrome Dev Editor I'm impressed - but generally, I don't think new languages should require new editors - better plugins for existing popular ones. I use Sublime, it's configured how I like, they can add the Dart AST for autocomplete etc to provide tab completion, renames, method lookups etc. if they want to.

We're working on exactly that - it involves making the Dart analysis engine run as a standalone server that that talk to different editors and provide code-completion, errors & warning, etc.