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

pkg/dev_compiler/lib/js/common/dart_sdk.js

Index: pkg/dev_compiler/lib/js/common/dart_sdk.js
diff --git a/pkg/dev_compiler/lib/js/common/dart_sdk.js b/pkg/dev_compiler/lib/js/common/dart_sdk.js
index e67f3b503706a17f55a42dd94d18c711793785f6..979392a9756ea55c05d0e603f73832b2db5a0fdf 100644
--- a/pkg/dev_compiler/lib/js/common/dart_sdk.js
+++ b/pkg/dev_compiler/lib/js/common/dart_sdk.js
@@ -1714,10 +1714,7 @@
         return null;
       }
     }
-    if (ret2 === dart.void) return true;
-    if (ret1 === dart.void) {
-      return ret2 === dart.dynamic || ret2 === async.FutureOr;
-    }
+    if (ret1 === dart.void) return dart._isTop(ret2);
     if (!dart._isSubtype(ret1, ret2, isCovariant)) return null;
     return true;
   };
@@ -1738,13 +1735,13 @@
     };
   };
   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;
@@ -2223,6 +2220,9 @@
     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);