Include the most recent updates on type modifiers in Dart.g

The class modifiers feature specification was updated such that mixins
cannot be `sealed`, and several other combinations were eliminated.
This CL makes those changes to Dart.g. The changes have been in effect
for a while (8 months since last update to the spec), but the update
to Dart.g was somehow not performed at the time. This CL fixes that
omission.

Change-Id: Ifd2124583a124cdaaa7822f94f70e707ec33b425
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/358460
Reviewed-by: William Hesse <[email protected]>
Commit-Queue: Erik Ernst <[email protected]>

Author: eernstg

tools/spec_parser/Dart.g

@@ -4,6 +4,8 @@
 
 // CHANGES:
 //
+// v0.39 Include latest changes to mixin related class modifiers.
+//
 // v0.38 Broaden `initializerExpression` to match implemented behavior.
 //
 // v0.37 Correct `libraryExport` to use `configurableUri`, not `uri`.
@@ -392,7 +394,7 @@ classDeclaration
     :    (classModifiers | mixinClassModifiers)
          CLASS typeWithParameters superclass? interfaces?
          LBRACE (metadata classMemberDeclaration)* RBRACE
-    |    classModifiers CLASS mixinApplicationClass
+    |    classModifiers MIXIN? CLASS mixinApplicationClass
     ;
 
 classModifiers
@@ -427,18 +429,11 @@ mixinApplicationClass
     ;
 
 mixinDeclaration
-    :    mixinModifier? MIXIN typeIdentifier typeParameters?
+    :    BASE? MIXIN typeIdentifier typeParameters?
          (ON typeNotVoidNotFunctionList)? interfaces?
          LBRACE (metadata mixinMemberDeclaration)* RBRACE
     ;
 
-mixinModifier
-    :    SEALED
-    |    BASE
-    |    INTERFACE
-    |    FINAL
-    ;
-
 // TODO: We might want to make this more strict.
 mixinMemberDeclaration
     :    classMemberDeclaration

tools/spec_parser/dart_spec_parser/Dart.g4

@@ -3,6 +3,9 @@
 // BSD-style license that can be found in the LICENSE file.
 
 // CHANGES:
+//
+// v0.40 Include latest changes to mixin related class modifiers.
+//
 // v0.39 Translate actions from Java to Dart.
 //
 // v0.38 Broaden `initializerExpression` to match implemented behavior.
@@ -397,7 +400,7 @@ classDeclaration
     :    (classModifiers | mixinClassModifiers)
          CLASS typeWithParameters superclass? interfaces?
          LBRACE (metadata classMemberDeclaration)* RBRACE
-    |    classModifiers CLASS mixinApplicationClass
+    |    classModifiers MIXIN? CLASS mixinApplicationClass
     ;
 
 classModifiers
@@ -432,18 +435,11 @@ mixinApplicationClass
     ;
 
 mixinDeclaration
-    :    mixinModifier? MIXIN typeIdentifier typeParameters?
+    :    BASE? MIXIN typeIdentifier typeParameters?
          (ON typeNotVoidNotFunctionList)? interfaces?
          LBRACE (metadata mixinMemberDeclaration)* RBRACE
     ;
 
-mixinModifier
-    :    SEALED
-    |    BASE
-    |    INTERFACE
-    |    FINAL
-    ;
-
 // TODO: We might want to make this more strict.
 mixinMemberDeclaration
     :    classMemberDeclaration