There is a lot of buzz around Dart recently, often questioning Google motivations and utility of Dart as replacement for JavaScript. I was searching for rationale of creating Dart rather than investing more effort in ECMAScript. In well known leaked mail its author is saying that

Javascript has historical baggage that cannot be solved without a clean break.

But there is only one concrete example given (apart of performance concerns) of "fundamental language problems", which is an

existence of a single Number primitive

So, my questions are:

1. How an existence of a single Number primitive can be a "fundamental problem"?

2. Are there other known "fundamental problems" in JavaScript?