Add enumEntry grammar update for augmentations

An `enumEntry` needs to allow for the keyword `augment` in order to
enable augmentation libraries. This CL performs a grammar rule update
in Dart.g and Dart.g4, such that this keyword can be included.

The corresponding change to the feature specification was performed in
https://github.com/dart-lang/language/pull/3688.

Change-Id: I6d6a3f09754a76da668dfc84cd20a7df812ecdbd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/360642
Reviewed-by: William Hesse <whesse@google.com>
Commit-Queue: Erik Ernst <eernst@google.com>

tools/spec_parser/Dart.g

@@ -4,6 +4,8 @@
 
 // CHANGES:
 //
+// v0.41 Add missing `enumEntry` update for augmentations.
+//
 // v0.40 Include support for augmentation libraries.
 //
 // v0.39 Include latest changes to mixin related class modifiers.
@@ -598,8 +600,9 @@ enumType
     ;
 
 enumEntry
-    :    metadata identifier argumentPart?
-    |    metadata identifier typeArguments? '.' identifierOrNew arguments
+    :    metadata AUGMENT? identifier argumentPart?
+    |    metadata AUGMENT? identifier typeArguments?
+         '.' identifierOrNew arguments
     ;
 
 typeParameter

tools/spec_parser/dart_spec_parser/Dart.g4

@@ -4,6 +4,8 @@
 
 // CHANGES:
 //
+// v0.42 Add missing `enumEntry` update for augmentations.
+//
 // v0.41 Include support for augmentation libraries.
 //
 // v0.40 Include latest changes to mixin related class modifiers.
@@ -604,8 +606,9 @@ enumType
     ;
 
 enumEntry
-    :    metadata identifier argumentPart?
-    |    metadata identifier typeArguments? '.' identifierOrNew arguments
+    :    metadata AUGMENT? identifier argumentPart?
+    |    metadata AUGMENT? identifier typeArguments?
+         '.' identifierOrNew arguments
     ;
 
 typeParameter