Asserts in initializers

[munificent]

Flutter likes it, so the language team considers the experiment a success and will ship asserts in constructor initialization lists in Dart 2.0.

Work remaining to be done:

• Support it in dart2js (Dart2js: Assert in initializer list. #30038)
• Support it in dartdevc (support assert in initializer list #27809)
• Support in front-end (Kernel should have an AssertInitializer node #30914)
• Specify it (Specify asserts in initialization lists #30967)
• Enable it by default and remove flag in VM (Remove flag and default --assert_initializer to enabled #30976)
• Enable it by default and remove flag in analyzer (Remove flag and turn on --enable-assert-initializers by default #30977)
• Document and announce it

[dgrove]

@munificent @kwalrath is there any more to do on the docs/announce front? If not, let's close this.

[kwalrath]

The language tour should cover asserts in initializers (and probably have more initializer coverage, in general). I've added this to dart-lang/site-www#507.

[munificent]

Yeah, aside from what Kathy said, I don't see any need to trumpet this feature from the heavens. It's pretty minor.

[rkj]

Any documentation? I just tried to use it, but cannot use foreach in the assert body, which makes it less usable than I would like (I wanted to verify that a map passed to the constructor doesn't have null values)