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

pkg/dev_compiler/lib/js/legacy/dart_sdk.js

Index: pkg/dev_compiler/lib/js/legacy/dart_sdk.js
diff --git a/pkg/dev_compiler/lib/js/legacy/dart_sdk.js b/pkg/dev_compiler/lib/js/legacy/dart_sdk.js
index cf5ca5dd0360f5148c5deb9103e1903cda459444..1fca021e87462ae6df7f6404e739b9bf5a0f4027 100644
--- a/pkg/dev_compiler/lib/js/legacy/dart_sdk.js
+++ b/pkg/dev_compiler/lib/js/legacy/dart_sdk.js
@@ -1715,10 +1715,6 @@ dart_library.library('dart_sdk', null, /* Imports */[
         return null;
       }
     }
-    if (ret2 === dart.void) return true;
-    if (ret1 === dart.void) {
-      return ret2 === dart.dynamic || ret2 === async.FutureOr;
-    }
     if (!dart._isSubtype(ret1, ret2, isCovariant)) return null;
     return true;
   };
@@ -1739,13 +1735,13 @@ dart_library.library('dart_sdk', null, /* Imports */[
     };
   };
   dart._isBottom = function(type) {
-    return type == dart.bottom;
+    return type == dart.bottom || type == core.Null;
   };
   dart._isTop = function(type) {
     if (dart.getGenericClass(type) === dart.getGenericClass(async.FutureOr)) {
       return dart._isTop(dart.getGenericArgs(type)[0]);
     }
-    return type == core.Object || type == dart.dynamic;
+    return type == core.Object || type == dart.dynamic || type == dart.void;
   };
   dart._isSubtype = function(t1, t2, isCovariant) {
     if (t1 === t2) return true;
@@ -2224,6 +2220,9 @@ dart_library.library('dart_sdk', null, /* Imports */[
     return false;
   };
   dart.is = function(obj, type) {
+    if (obj == null) {
+      return type == core.Null || dart._isTop(type);
+    }
     let result = dart.strongInstanceOf(obj, type);
     if (result !== null) return result;
     let actual = dart.getReifiedType(obj);