Most (all?) of the improvements listed in the document are already present in Dart. You can use them today. Good to see that these may also filter into JS in a few years time.

Google is capable on working on improving JS and Dart simultaneously. You can see Dart as a test ground for features that may make it to JS. Cross-browser improvements made to JS will also benefit the Dart ecosystem, as it is a compile to JS platform.

Don't expect Dart to disappear any time soon. Despite popular rhetoric, Google actually has a good history of supporting its web programming platforms, for example GWT and Closure.