dart2js: target modern browsers and use modern JS

[sigmundch]

Two years ago we announced a plan to deprecate support for IE11, but due to change in priorities we have not yet made the underlying changes.

This bug is to track any work to enable modern JS features and start using them in our compiler. This includes:

• [x] increase test coverage of --no-legacy-javascript
• [x] fix current failures with the flag
• [ ] explore other features we'd like to include in an initial version of this flag, this may include:
  • [x] syntax changes (e.g. method syntax has come up, but it requires careful integration with internal stack deobfuscation tools)
  • [ ] use of symbols for various reasons (e.g. today we feature detect to use it in rti)
  • [ ] Use of ES6 maps
  • [ ] Use ES6 math functions (#29623)
  • [ ] Removing IE11 polyfills in dart:html
• [x] measure performance and code-size changes
• [x] dogfood the flag on a couple internal apps
• [x] enable the flag by default

[lexaknyazev]

Please explore including #29623 in this plan.

[sigmundch]

Thanks for the suggestion, I went ahead and added it to the list above.

[ghost]

it is very sad that you have to adapt to a poor javascript system, instead of developing your own virtual machine for the browser

[lexaknyazev]

Please consider promoting BigInt into language. With modern JS, it implies:

• Literals and const values: #33893
• JS-interop (to avoid workarounds)
• 64-bit typed data via BigInt64Array / BigUint64Array

[MrBirb]

I believe in 2.14 this is the default behavior?

[sigmundch]

Yes, the flag was flipped by default in 2.14. So far, though, we only use ES6 method syntax, which was the change that was providing the biggest win in terms of code-size reduction (as much as 4% on some of our large internal apps).