Update language tour for Dart 2 changes

[kwalrath]

• Implicit new and const - lrn (Optional new and const for Dart 2 (meta issue) sdk#30921) (partially fixed in Dart 2 changes to the language tour #677)
• Support assert in initializers - rnystrom (Asserts in initializers sdk#30968) (fixed in Dart 2 changes to the language tour #677)
• Fixed-sized integers limited to 64 bits - floitsch (Limit integers to 64 bits. sdk#28595, issue 64-bit integers in the language tour #591 ) (fixed in Dart 2 changes to the language tour #677)
• Generalized void - eernst (Support generalized void sdk#30176) (fixed [partially?] in Dart 2 changes to the language tour #677)
• Eliminate fuzzy arrows - leafp (Eliminate fuzzy arrows sdk#29630)
• Start async functions synchronously - floitsch (fixed in Dart 2 changes to the language tour #677)
• @Proxy & noSuchMethod (issue Update noSuchMethod explanation for Dart 2 #442, pr Update language tour: Dart 2 changes to @proxy & noSuchMethod #511)
• First arg to assert can't be a function (issue In Dart 2, first arg to assert() can't be a function #572)
• Introduce generic syntax earlier (issue Introduce generics earlier in the language tour #588) (fixed in Dart 2 changes to the language tour #677)
• 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: #844 (comment). 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: #844 (comment). 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.