[dart:html] Change late finals to external getters

Late final fields in the JS classes in js_typed_interop tests are
changed to external getters instead.

Change-Id: I5ed17e18b5fc59bd7267ac5f8ebda649c6fb21e5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/142722
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Commit-Queue: Srujan Gaddam <srujzs@google.com>

tests/lib/html/js_typed_interop_type2_test.dart

@@ -12,7 +12,7 @@ import 'package:expect/expect.dart';
 @JS()
 @anonymous
 class C {
-  late final foo;
+  external get foo;
 
   external factory C({required foo});
 }
@@ -20,7 +20,7 @@ class C {
 @JS()
 @anonymous
 class D {
-  late final foo;
+  external get foo;
 
   external factory D({required foo});
 }

tests/lib/html/js_typed_interop_type3_test.dart

@@ -19,7 +19,7 @@ class A {
 @JS()
 @anonymous
 class C {
-  late final foo;
+  external get foo;
 
   external factory C({required foo});
 }
@@ -27,7 +27,7 @@ class C {
 @JS()
 @anonymous
 class D {
-  late final foo;
+  external get foo;
 
   external factory D({required foo});
 }