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.