Add informal specification of asserts in initializer lists.

docs/language/informal/assert-in-initializer-list.md

+# Asserts in Initializer List
+[lrn@google.com](mailto:lrn@google.com)
+Version 1.1 (2017-06-08)
+Status: Accepted, Informally specified
+
+(See: http://dartbug.com/24841, http://dartbug.com/27141)
+
+In some cases, you want to validate your inputs before creating an instance, even in a const constructor. To allow that, we have tested the possibility of allowing assert statements in the initializer list of a generative constructor.
+
+We started by implementing the feature in the VM behind a flag, with at syntax support from the analyzer and the formatter.
+
+This was as successful experiment, and the feature is actively being used by the Flutter project, so now we promote the experimental feature to a language feature.
+
+## Syntax
+
+The syntax is changed to allow an assert statement without trailing semicolon (just the `assert(condition[, message])`) to appear as an item in the initializer list.
+Example:
+
+```dart
+   C(x, y) : this.foo = x, assert(x < y), this.bar = y;
+```
+
+The assert can occur anywhere in the list where an initializing assignment can.
+
+That is, the grammar changes so that *superCallOrFieldIntitializer* can also produce an assert.
+
+For simplicity, we add a new production for the assert-without-the-semicolon, and reuse that in both the initializer list and the *assertStatement*.
+
+> *superCallOrFieldInitializer*:
+> &nbsp;&nbsp;&nbsp; **super** arguments
+> &nbsp;&nbsp;| **super** ‘.’ identifier arguments
+> &nbsp;&nbsp;| fieldInitializer
+> &nbsp;&nbsp;| assertion
+> &nbsp;&nbsp;;
+>
+> assertion:  **assert** ‘(' expression (‘,' expression)? ‘)'  ;
+>
+> assertStatement: assertion ‘;' ;
+
+The *superCallOrFieldInitializer* production will probably change name too, perhaps to *initializerListEntry*, but that's not important for the behavior.
+
+## Semantics
+
+The initializer list assert works the same way as an assert statement in a function body (with special treatment for asserts in a const constructor's initializer list, see next section). The assert expressions are evaluated in the initializer list scope, which does not have access to `this`, exactly the same way that an assert statement would be evaluated in the same scope. The runtime behavior is effectively:
+
+1.  evaluate the condition expression (in the initializer list scope) to a result, `o`.
+1.  If `o` implements `Function`, call it with zero arguments and let `r` be the return value,
+1.  otherwise let `r` = `o`.
+1.  Perform boolean conversion on `r`. This throws if `r` is not an instance of `bool`.
+1.  if `r` isn't `true`,
+    a.  if there is a message expression, evaluate that to a value `m`
+    b.  otherwise let `m` be `null`
+    c.  then throw an `AssertionError` with `m` as message.
+
+Statically, like in an assertion statement, it's a warning if the static type of the condition expression isn't assignable to either `bool` or `bool Function()`.
+
+Here step 2, 4 and 5a may throw before reaching step 5c, in which case that is the effect of the assert.
+
+
+The assert statement is evaluated at its position in the initializer list, relative to the left-to-right evaluation of initializer list entries.
+
+As usual, assert statements have no effect unless asserts are enabled (e.g., by running in checked mode).
+
+
+## Const Semantics
+
+If the constructor is a const constructor, the condition and message expressions in the assert must be potentially compile-time constant expressions. If any of them aren't, it is a compile-time error, the same way a non-potentially compile-time constant initializer expression in the initializer list is.
+
+Further, the condition expression should not evaluate to a function, since we can't call functions at compile time. We can't prevent it from evaluating to a function, but the function cannot not be called. To account for this, the behavior above is changed for const constructor initializer list asserts:
+
+*Step 2 above is dropped for an assert in a const constructor initializer list.*
+
+The change is entirely syntax driven - an assert inside a const constructor initializer list does not test whether the expression is a function, not even when the constructor is invoked using `new`.
+This change from the current specification is needed because asserts previously couldn't occur in a (potentially) const context[^1].
+
+During a const constructor invocation (that is, when the const constructor is invoked using the `const` prefix), if the assert fails, either due to boolean conversion when `r` is not a boolean value or due to assertion failure when `r` is `false`, it is treated like any other compile-time throw in a compile-time constant expression, and it causes a compile-time error.
+
+
+## Revisions
+
+1.0 (2016-06-23) Initial specification.
+
+1.1 (2017-06-08) Handle second expression in asserts as well, add grammar rules.
+
+
+## Notes
+
+[^1]:
+     If we ever add "const functions" which can be "called" in a const context, then we may allow them here, but other functions are still compile time errors.