> Complaining about JS as a backward-compatibility-constrained language, to support the idea that JS uplift needs expensive, separate "REPLACE JS" alterna-language/runtime projects such as Dart, which inevitably -- because you can't replace JS, flamebait trigger warning LOL -- backpeddle to compile-to-JS tooling and tardy tech transfer into ECMA-262, doesn't justify the opportunity cost.

Ok, let's say JS continues to evolve over the next 5-10 years, and by this time all browsers support integers, optional types, operator overloading, less implicit conversion, and mixins. By this point compiling Dart to JS is trivial, and the cross compiled code is also completely readible. So I can take my existing Dart project and compile it to ES11, clean up a few things by hand, and then continue developing in ES11. In the meantime I've had 5-10 years being very productive developing in Dart with good tool support. This seems like a win to me.

Having a VM is also incredibly beneficial for development and debugging. These points alone justify investment in Dart.

But there are more benefits. Providing a testing ground for VM engineers to try out new features without legacy compatibility constraints. They haven't really got started on performance yet, and they're already solidly ahead of V8. This can provide insights for TC39, to see what kind of performance/memory improvements are possible if changes are made to the language.

> Bignums

I'm aware of the bignums strawman, and I'm aware the page on the wiki dates to 2010. But I repeat: even if the V8 team had implemented bignums behind a flag in 2010, do you really think that the other vendors (including MS and Apple) would have also implemented them already? I find this scenario unlikely. (But kudos to MS/A for implementing the new ES6 features. I'm glad to see JS evolving)

> [Snapshotting] could be added to JS if it could be done for Dart.

My understanding was that the V8 team actually implemented snapshotting and initially used it for loading their JS core library. They didn't think it was possible to use it for loading cached web code. Not because of the globals object, but because javascript code must be executed to build up classes, i.e. setting prototypes etc. Because execution and structure is interleaved, top level code and initialisors could perform side effects before the static program structure exists.

> there hasn't been much uplift over five years.

It looks like Dart is a longer term bet. It will be interesting to see what kind of an influence it has over the next decade or so. Especially given the recent talk around adding optional typing to JS. The Dart team are also currently experimenting with co-operative threading, growable stacks and concurrency primitives. https://github.com/dart-lang/fletch/wiki/Coroutines-and-Thre...

> My point about opportunity costs stands. We could have come a lot farther, faster.

I'm not convinced. I don't see how the development resourcing on the V8 development team could have sped up the ES6 standards process. The ES6 process includes getting consesus with Apple/Microsoft - this process isn't time constrained by V8 development resources.

Consider the opportunity cost if they chose not to develop Dart. Developers have to wait a lot longer to get modern tooling, with: completions, doc hovers, code navigation and static analysis. These really make a huge difference to developer productivity. All of the experience gained here can feed back into javascript tooling (assuming optional typing makes it into ES).

Also consider the individual developer's motivations. It's great that you are excited about working on JS and spidermonkey year after year. But what were the motivations of the developers at google? If the developers are passionate about the web like yourself, then they can continue to work with the V8 team. But if what excites them, is working on a state of the art dynamically typed VM, well then removing some of the legacy JS constraints allows them freedom to innovate. If their only choice had been to work on V8, they could well leave the company and work on another VM project somewhere else. So what would the cost have been if google management said no you can't do Dart?

> You switched from "Dart does good for the Web via (eventual) JS uplift" (paraphrasing), to "Dart just needs to provide good development tooling" ... What's with the goalpost moving ... ?

The shifted goal post was: "Dart ain't gonna replace JS in the foreseeable future". I never claimed that Dart will, or needs to, replace javascript.