[CFE/kernel] Fix relink bug with ExtensionTypeDeclaration

Over the weekend the flutter-frontend bot started failing.
This was caused by an update on the flutter side, where a dependency
started using extension types, and revealed a bug with relinking on our
side.
This CL fixes the bug which should make the bot succeed again (as well
as, I think, fix a potential leak/failure if rejecting an advanced
invalidated incremental compilation.
I will add tests in an upcoming CL.

Change-Id: I1e73314b5ce5fe70819f921f9b176c7286b31f8a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/350801
Reviewed-by: Chloe Stefantsova <[email protected]>
Commit-Queue: Jens Johansen <[email protected]>

Author: jensjoha

pkg/kernel/lib/ast.dart

@@ -574,7 +574,7 @@ class Library extends NamedNode
     for (int i = 0; i < extensionTypeDeclarations.length; ++i) {
       ExtensionTypeDeclaration extensionTypeDeclaration =
           extensionTypeDeclarations[i];
-      extensionTypeDeclaration._relinkNode();
+      extensionTypeDeclaration.relink();
     }
   }
 
@@ -1909,6 +1909,21 @@ class ExtensionTypeDeclaration extends NamedNode implements TypeDeclaration {
     _procedures = procedures;
   }
 
+  /// This is an advanced feature. Use of this method should be coordinated
+  /// with the kernel team.
+  ///
+  /// See [Component.relink] for a comprehensive description.
+  ///
+  /// Makes sure all references in named nodes in this extension type
+  /// declaration points to said named node.
+  void relink() {
+    this.reference.node = this;
+    for (int i = 0; i < procedures.length; ++i) {
+      Procedure member = procedures[i];
+      member._relinkNode();
+    }
+  }
+
   @override
   R accept<R>(TreeVisitor<R> v) => v.visitExtensionTypeDeclaration(this);