fix #28642, handling of top and bottom types in DDC

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/types.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// This library defines the representation of runtime types.
 part of dart._runtime;
 
 final metadata = JS('', 'Symbol("metadata")');
 
 /// The symbol used to store the cached `Type` object associated with a class.
@@ skipping 638 matching lines @@
       return ($isCovariant) ? false : null;
     }
     if (!$_isSubtype(n2, n1, !$isCovariant)) {
       return null;
     }
   }
 
   // Check return type last, so that arity mismatched functions can be
   // definitively rejected.
 
-  // We allow any type to subtype a void return type, but not vice versa
-  if (ret2 === $_void) return true;
-  // Dart allows void functions to subtype dynamic functions, but not
-  // other functions.
-  // TODO(jmesserly): this check does not match our compile time subtype
-  // implementation. Reconcile.
-  if (ret1 === $_void) {
-    return ret2 === $dynamic || ret2 === $FutureOr;
-  }
+  // For `void` we will give the same answer as the VM, so don't return null.
+  if (ret1 === $_void) return $_isTop(ret2);
+
   if (!$_isSubtype(ret1, ret2, $isCovariant)) return null;
   return true;
 })()''');
 
 /// TODO(leafp): This duplicates code in operations.dart.
 /// I haven't found a way to factor it out that makes the
 /// code generator happy though.
 _subtypeMemo(f) => JS(
     '',
     '''(() => {
@@ skipping 13 matching lines @@
   };
 })()''');
 
 /// Returns true if [t1] <: [t2].
 /// Returns false if [t1] </: [t2] in both spec and strong mode
 /// Returns undefined if [t1] </: [t2] in strong mode, but spec
 ///  mode may differ
 final isSubtype = JS(
     '', '$_subtypeMemo((t1, t2) => (t1 === t2) || $_isSubtype(t1, t2, true))');
 
-_isBottom(type) => JS('bool', '# == #', type, bottom);
+_isBottom(type) => JS('bool', '# == # || # == #', type, bottom, type, Null);
 
 _isTop(type) {
   if (JS('bool', '# === #', getGenericClass(type), getGenericClass(FutureOr))) {
     return _isTop(JS('', '#[0]', getGenericArgs(type)));
   }
-  return JS('bool', '# == # || # == #', type, Object, type, dynamic);
+  return JS('bool', '# == # || # == # || # == #',
+      type, Object, type, dynamic, type, _void);
 }
 
 _isSubtype(t1, t2, isCovariant) => JS(
     '',
     '''(() => {
   if ($t1 === $t2) return true;
 
   // Trivially true.
   if ($_isTop($t2) || $_isBottom($t1)) {
     return true;
@@ skipping 152 matching lines @@
     return true;
   }
 
   let typeArgs = $getGenericArgs($type);
   if (!typeArgs) return true;
   for (let t of typeArgs) {
     if (t != $Object && t != $dynamic) return false;
   }
   return true;
 })()''');