[dyn_modules] Support const constructors in extension types

Such constructors are desugared to top-level Procedures marked as
const.

TEST=pkg/dynamic_modules/test/data/extension_type2

Change-Id: I131f720249292e77a9ec7010aaefcde550605bd9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/463500
Reviewed-by: Sigmund Cherem <[email protected]>
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Alexander Markov <[email protected]>

Author: alexmarkov

pkg/dynamic_modules/test/data/extension_type2/main.dart

@@ -6,7 +6,9 @@ import '../../common/testing.dart' as helper;
 import 'package:expect/expect.dart';
 
 void main() async {
-  final result = (await helper.load('entry1.dart')) as int;
-  Expect.equals(1, result);
+  final result = (await helper.load('entry1.dart')) as List;
+  Expect.equals(1, result[0] as int);
+  Expect.equals(52, (result[1] as int Function(int)).call(10));
+  Expect.equals(53, (result[2] as int Function(int)).call(10));
   helper.done();
 }

pkg/dynamic_modules/test/data/extension_type2/modules/entry1.dart

@@ -5,6 +5,10 @@
 import '../shared/shared.dart';
 
 @pragma('dyn-module:entry-point')
-Object? dynamicModuleEntrypoint() => method(value);
+Object? dynamicModuleEntrypoint() => [
+  method(value),
+  const C.foo42(),
+  const C.foo43(),
+];
 
 int method(B b) => b.foo();

pkg/dynamic_modules/test/data/extension_type2/shared/shared.dart

@@ -9,3 +9,10 @@ B get value => B(A());
 extension type B(A _internal) {
   int foo() => 1;
 }
+
+extension type const C(int Function(int) raw) {
+  const C.foo42() : this(_foo42);
+  const C.foo43() : this(foo43);
+}
+
+int _foo42(int x) => x + 42;

pkg/dynamic_modules/test/data/extension_type2/shared/shared_a.dart

@@ -3,3 +3,5 @@
 // BSD-style license that can be found in the LICENSE file.
 
 class A {}
+
+int foo43(int x) => x + 43;

pkg/front_end/lib/src/util/trim.dart

@@ -212,6 +212,8 @@ class Trimmer extends RecursiveVisitor {
 
   @override
   void visitProcedure(Procedure node) {
+    // Keep bodies of const factories.
+    if (node.isConst) return;
     // Preserve method bodies of mixin declarations, these are copied when
     // mixins are applied in subtypes.
     if (!preserveMemberBodies) {

pkg/vm/lib/transformations/dynamic_interface_annotator.dart

@@ -262,6 +262,7 @@ class _Annotator extends RecursiveVisitor {
         annotateMember(tearOff as Member);
       }
     }
+    node.declaredRepresentationType.accept(this);
   }
 }
 
@@ -397,6 +398,9 @@ class _ImplicitUsesAnnotator extends RecursiveVisitor {
       final target = node.function.redirectingFactoryTarget?.target;
       target?.acceptReference(this);
     }
+    if (node.isConst) {
+      node.visitChildren(this);
+    }
   }
 
   @override