Adjust Dart.g such that an extension can not have the name type

The declaration `extension type on(int i,) {}` is ambiguous: It can
be an extension named `type` or an extension type named `on`. This
CL adjusts the specification grammar Dart.g such that it is
disambiguated in favor of being an extension type.
See dart-lang/language#3258 for details.

Change-Id: I74f800a75c6c5061675c345c546a85cff046d27f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/332243
Reviewed-by: William Hesse <[email protected]>
Commit-Queue: Erik Ernst <[email protected]>

Author: eernstg

tools/spec_parser/Dart.g

@@ -460,7 +460,7 @@ extensionTypeMemberDeclaration
     ;
 
 extensionDeclaration
-    :    EXTENSION identifier? typeParameters? ON type
+    :    EXTENSION typeIdentifierNotType? typeParameters? ON type
          LBRACE (metadata extensionMemberDefinition)* RBRACE
     ;
 
@@ -1046,13 +1046,18 @@ qualifiedName
     |    typeIdentifier '.' typeIdentifier '.' identifierOrNew
     ;
 
-typeIdentifier
+typeIdentifierNotType
     :    IDENTIFIER
     |    DYNAMIC // Built-in identifier that can be used as a type.
-    |    otherIdentifier // Occur in grammar rules, are not built-in.
+    |    otherIdentifierNotType // Occur in grammar rules, are not built-in.
     |    { asyncEtcPredicate(getCurrentToken().getType()) }? (AWAIT|YIELD)
     ;
 
+typeIdentifier
+    :    typeIdentifierNotType
+    |    TYPE
+    ;
+
 typeTest
     :    isOperator typeNotVoid
     ;
@@ -1637,7 +1642,7 @@ builtInIdentifier
     |    TYPEDEF
     ;
 
-otherIdentifier
+otherIdentifierNotType
     :    ASYNC
     |    BASE
     |    HIDE
@@ -1646,10 +1651,14 @@ otherIdentifier
     |    SEALED
     |    SHOW
     |    SYNC
-    |    TYPE
     |    WHEN
     ;
 
+otherIdentifier
+    :    otherIdentifierNotType
+    |    TYPE
+    ;
+
 // ---------------------------------------- Lexer rules.
 
 fragment