Extension types. Report an error if the feature is not enabled.

Change-Id: Icc1e4fddea91dab22fcab694928c9138b0781e76
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/318440
Reviewed-by: Phil Quitslund <pquitslund@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/lib/src/fasta/ast_builder.dart

@@ -1658,15 +1658,22 @@ class AstBuilder extends StackListener {
     final representation = pop() as RepresentationDeclarationImpl;
     final constKeyword = pop() as Token?;
 
-    final builder = _classLikeBuilder as _ExtensionTypeDeclarationBuilder;
-    declarations.add(
-      builder.build(
-        typeKeyword: typeKeyword,
-        constKeyword: constKeyword,
-        representation: representation,
-        implementsClause: implementsClause,
-      ),
-    );
+    if (enableInlineClass) {
+      final builder = _classLikeBuilder as _ExtensionTypeDeclarationBuilder;
+      declarations.add(
+        builder.build(
+          typeKeyword: typeKeyword,
+          constKeyword: constKeyword,
+          representation: representation,
+          implementsClause: implementsClause,
+        ),
+      );
+    } else {
+      _reportFeatureNotEnabled(
+        feature: ExperimentalFeatures.inline_class,
+        startToken: typeKeyword,
+      );
+    }
 
     _classLikeBuilder = null;
   }

pkg/analyzer/test/src/dart/parser/extension_type_test.dart

@@ -65,6 +65,34 @@ ExtensionTypeDeclaration
 ''');
   }
 
+  test_featureNotEnabled() {
+    final parseResult = parseStringWithErrors(r'''
+// @dart = 3.1
+class A {}
+extension type B(int it) {}
+class C {}
+''');
+    parseResult.assertErrors([
+      error(ParserErrorCode.EXPERIMENT_NOT_ENABLED, 36, 4),
+    ]);
+
+    final node = parseResult.findNode.unit;
+    assertParsedNodeText(node, r'''
+CompilationUnit
+  declarations
+    ClassDeclaration
+      classKeyword: class
+      name: A
+      leftBracket: {
+      rightBracket: }
+    ClassDeclaration
+      classKeyword: class
+      name: C
+      leftBracket: {
+      rightBracket: }
+''');
+  }
+
   test_field_metadata() {
     final parseResult = parseStringWithErrors(r'''
 extension type A(@foo int it) {}