[ddc] Compare types with new DartTypeEquivalence

Avoids an assertion failure in the front end.

Change-Id: I7994fa611f82011857750cc60af64a2c89c96bf7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/152923
Reviewed-by: Dmitry Stefantsov <dmitryas@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>

pkg/dev_compiler/lib/src/kernel/compiler.dart

@@ -12,6 +12,7 @@ import 'package:kernel/kernel.dart' hide MapEntry;
 import 'package:kernel/library_index.dart';
 import 'package:kernel/type_algebra.dart';
 import 'package:kernel/type_environment.dart';
+import 'package:kernel/src/dart_type_equivalence.dart';
 import 'package:source_span/source_span.dart' show SourceLocation;
 import 'package:path/path.dart' as p;
 
@@ -5331,7 +5332,8 @@ class ProgramCompiler extends ComputeOnceConstantVisitor<js_ast.Expression>
     }
 
     if (!isTypeError &&
-        from.withDeclaredNullability(Nullability.nonNullable) == to &&
+        DartTypeEquivalence(_coreTypes, ignoreTopLevelNullability: true)
+            .areEqual(from, to) &&
         _mustBeNonNullable(to)) {
       // If the underlying type is the same, we only need a null check.
       return runtimeCall('nullCast(#, #)', [jsFrom, _emitType(to)]);