Site-www: Update language tour for Dart 2 changes

[kwalrath]

• [x] Implicit new and const - lrn (dart-lang/sdk#30921) (partially fixed in #677)
• [x] Support assert in initializers - rnystrom (dart-lang/sdk#30968) (fixed in #677)
• [x] Fixed-sized integers limited to 64 bits - floitsch (dart-lang/sdk#28595, issue #591 ) (fixed in #677)
• [x] Generalized void - eernst (dart-lang/sdk#30176) (fixed [partially?] in #677)
• [ ] Eliminate fuzzy arrows - leafp (dart-lang/sdk#29630)
• [x] Start async functions synchronously - floitsch (fixed in #677)
• [x] @proxy & noSuchMethod (issue #442, pr #511)
• [x] First arg to assert can't be a function (issue #572)
• [x] Introduce generic syntax earlier (issue #588) (fixed in #677)
• [x] Asserts in initializers

[kwalrath]

I'm working on the strong mode changes, which are pretty invasive.

[sethladd]

This is so awesome to see!

[kwalrath]

I don't know how to update the language tour for fuzzy arrows, so I'll be contacting @leafpetersen. We expect to start using implicit new/const everywhere for beta 3.

[kwalrath]

We should cover asserts in initializers (and probably beef up the initializer coverage a bit).

[kwalrath]

I'm starting on removing new and const.

[chalin]

I'm starting on removing new and const.

There is a PR pending for that: _Remove all uses of "new" and "const" from "Effective Dart"_ #844. In particular, note this comment: https://github.com/dart-lang/site-www/pull/844#issuecomment-395498524. If we have the green light to go ahead with this, then great!

[kwalrath]

I'm starting on removing new and const.

There is a PR pending for that: _Remove all uses of "new" and "const" from "Effective Dart"_ #844. In particular, note this comment: https://github.com/dart-lang/site-www/pull/844#issuecomment-395498524. If we have the green light to go ahead with this, then great!

Because Flutter's style (at least within constructors) is not to use new, I've been asked to avoid new whenever possible. The bar for Effective Dart is higher, because it sets conventions used everywhere. (I suppose you could say the same for the language tour, but it _does_ cover new, and I don't think people copy-paste its code as much as they do from more focused docs.)

[kwalrath]

Implicit new and const are covered. I don't _think_ we need to cover fuzzy arrows, but I should probably still add an example of an assert in an initializer.

[kwalrath]

677 added an example of using assert in an initializer.