Adjusted status indications on feature specs

The Dart 1 specification of 'generic method syntax' had no status
indicator, it is now marked as 'background material' because generic
methods (in full) have been integrated into the language specification.
Several other changes of a similar nature.

Implication: It should now be possible for readers of these documents
to trust their status indication. As we go, we will need to update them
again, especially when something is integrated into the language
specification.

Do we want to migrate background material to another directory? Do we
want to rename the directory `informal` to `feature` or somesuch?

Change-Id: Ia3851bdbe7b5a46d71848c376906f95feb4db349
Reviewed-on: https://dart-review.googlesource.com/34663
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>

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

@@ -1,7 +1,11 @@
 # Asserts in Initializer List
 [lrn@google.com](mailto:lrn@google.com)
+
 Version 1.1 (2017-06-08)
-Status: Accepted, Informally specified
+
+**Status**: Integrated into the language specification in
+[`609d26a`](https://github.com/dart-lang/sdk/commit/609d26a2274ccde0f74725f4df7e081ebc8ea020);
+this document is now background material.
 
 (See: http://dartbug.com/24841, http://dartbug.com/27141)
 

docs/language/informal/covariant-overrides.md

@@ -1,10 +1,10 @@
 # Covariant Overrides
 
-Owner: rnystrom@, eernstg@.
+**Owner**: rnystrom@, eernst@.
 
-Status: Implemented.
+**Status**: Implemented.
 
-Version: 1.1 (Oct 10, 2017).
+**Version**: 1.1 (Oct 10, 2017).
 
 ## Summary
 

docs/language/informal/generalized-void.md

@@ -1,6 +1,6 @@
 ## Feature: Generalized Void
 
-Author: eernst@
+**Author**: eernst@
 
 **Status**: Under implementation.
 

docs/language/informal/generic-method-syntax.md

@@ -1,5 +1,12 @@
 # Feature: Generic Method Syntax
 
+**Author**: eernst@
+
+**Status**: Integrated into (and subsumed by) updates to the language
+specification as of
+[`673d5f0`](https://github.com/dart-lang/sdk/commit/673d5f0a665085153d25f8c39495eacdb010ca64).
+This document is now background material.
+
 **This document** is an informal specification of the support in Dart 1.x
 for generic methods and functions which includes syntax and name
 resolution, but not reification of type arguments.

docs/language/informal/int64.md

@@ -1,7 +1,11 @@
 Dart - Fixed-Size Integers
 ===
-2017-09-26
-floitsch@google.com
+
+**Author**: floitsch@
+
+**Version**: 2017-09-26.
+
+**Status**: Under implementation.
 
 This document discusses Dart's plan to switch the `int` type so that it represents 64-bit integers instead of bigints. It is part of our continued effort of changing the integer type to fixed size ([issue]).
 

docs/language/informal/mixin-declaration.md

@@ -1,10 +1,10 @@
 # Dart 2.0 Mixins
 
-Author: [lrn@google.com](mailto:lrn@google.com)
+**Author**: [lrn@google.com](mailto:lrn@google.com)
 
-Version 0.6 (2017-06-14)
+**Version**: 0.6 (2017-06-14)
 
-Status: Mostly designed, ready for external comments.
+**Status**: Mostly designed, ready for external comments.
 
 ## Proposal
 This proposal introduces a new syntax for declaring mixins, separate from deriving a mixin from a class declaration. It expects to deprecate and remove the ability to derive a mixin from a class declaration, but doesn't require it.

docs/language/informal/optional-new-const.md

@@ -1,10 +1,11 @@
 # Optional new/const
 
-Author: Lasse R.H. Nielsen ([lrn@google.com](mailto:lrn@google.com))
+**Author**: Lasse R.H. Nielsen ([lrn@google.com](mailto:lrn@google.com))
 
-Version: 0.8 (2017-06-20)
+**Version**: 0.8 (2017-06-20)
 
-Status: Under discussion
+**Status**: This is background material for
+[implicit-creation.md](https://github.com/dart-lang/sdk/blob/master/docs/language/informal/implicit-creation.md).
 
 This informal specification documents a group of four related features.
 * Optional `const`

docs/language/informal/super-bounded-types.md

@@ -4,7 +4,7 @@
 
 **Version**: 0.5 (2018-01-11).
 
-**Status**: Under discussion.
+**Status**: Under implementation.
 
 **This document** is an informal specification of the support in Dart 2 for
 using certain generic types where the declared bounds are violated. The