Support for Null as bottom in dart2js

sdk/lib/_internal/js_runtime/lib/js_rti.dart

Index: sdk/lib/_internal/js_runtime/lib/js_rti.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/js_rti.dart b/sdk/lib/_internal/js_runtime/lib/js_rti.dart
index 0ce3e69993d74324f3a1466f1ec964b5bff11188..91254ef2dad1901511974373cb340ff30e9d1793 100644
--- a/sdk/lib/_internal/js_runtime/lib/js_rti.dart
+++ b/sdk/lib/_internal/js_runtime/lib/js_rti.dart
@@ -448,6 +448,7 @@ bool isSubtype(var s, var t) {
   if (isIdentical(s, t)) return true;
   // If either type is dynamic, [s] is a subtype of [t].
   if (s == null || t == null) return true;
+  if (isNullType(s)) return true;
   if (isDartFunctionType(t)) {
     return isFunctionSubtype(s, t);
   }