Removed permission to do `voidExpression is T` and `.. is! T`.

docs/language/informal/generalized-void.md

 ## Feature: Generalized Void
 
 Author: eernst@
 
 **Status**: Under implementation.
 
 **This document** is an informal specification of the generalized support
 in Dart 1.x for the type `void`. Dart 2 will have a very similar kind of
 generalized support for `void`, without the function type subtype exception
 that this feature includes for backward compatibility in Dart 1.x. This
@@ skipping 252 matching lines @@
 able to treat the type void as `Object` in all cases during subtype
 checks.*
 
 It is a static warning for an expression to have type void, except for the
 following situations:
 
 *   In an expressionStatement `e;`, e may have type void.
 *   In the initialization and increment expressions of a for-loop,
     `for (e1; e2; e3) {..}`, `e1` and `e3` may have type void.
 *   In a typeCast `e as T`, `e` may have type void.
-*   In a typeTest `e is T` or `e is! T`, `e` may have type void.
 *   In a parenthesized expression `(e)`, `e` may have type void.
 *   In a return statement `return e;`, when the return type of the innermost
     enclosing function is the type void, `e` may have type void.
 
 *Note that the parenthesized expression itself has type void, so it is
 again subject to the same constraints. Also note that we may not allow
 return statements returning an expression of type void in the future, but
 it is allowed here for backward compatibility.*
 
 During bounds checking, it is possible that a bound of a formal type
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 this would presumably require a transitive traversal of all generic classes
 and functions where the corresponding formal type parameter is passed on to
 other generic classes or functions, which would be highly brittle: A tiny
 change to a generic class or function could break code far away. So we do
 not wish to prevent formal type parameter bounds from indirectly becoming
 the type void. This motivated the decision to treat such a void-valued
 bound as `Object`.
 
 ## Updates
 
+*   August 16h 2017: Removed exceptions allowing `e is T` and `e is! T`.
+
 *   August 9th 2017: Transferred to SDK repo, docs/language/informal.
 
 *   July 16th 2017: Reformatted as a gist.
 
 *   June 13th 2017: Compile-time error for using a void value was changed to
     static warning.
 *   June 12th 2017: Grammar changed extensively, to use
     `typeNotVoid` rather than
     `voidOrType`.
 *   June 5th 2017: Added `typeCast` and
     `typeTest` to the locations where void
     expressions may occur.