add a restriction that augmented types must duplicate all keywords (#2959)

Author: jakemac53

working/augmentation-libraries/feature-specification.md

@@ -1,6 +1,7 @@
 # Augmentation Libraries
 
-Author: [email protected], Version: 1.6 (see [Changelog](#Changelog) at end)
+Author: [email protected], [email protected]
+Version: 1.7 (see [Changelog](#Changelog) at end)
 
 Augmentation libraries allow splitting a Dart library into files. Unlike part
 files, each augmentation has its [own imports][part imports] and top-level
@@ -292,6 +293,11 @@ This means that instead of creating a new type declaration, the augmentation
 modifies a corresponding declaration in the main library or one of its other
 augmentations.
 
+All the keywords (other than `augment`) must be identical between the original
+declaration and the augmentation. This is to ensure that looking at either one
+will give a complete depiction of the capabilities of the type, and an
+augmentation cannot introduce hidden restrictions.
+
 A class or enum augmentation may specify `implements` and `with` clauses. When
 those appear, the specified superinterfaces and mixins are appended to the main
 class's superinterface and mixin lists, respectively.
@@ -798,6 +804,11 @@ language and our tools.
 
 ## Changelog
 
+### 1.7
+
+*   Specify that augmentations must contain all the same keywords as the
+    original declaration (and no more).
+
 ### 1.6
 
 *   Allow class augmentations to use different names for type parameters. This