Convert a == to a builtin identity check if the receiver type can only hit Object==.

sdk/lib/_internal/compiler/implementation/universe/universe.dart

Index: sdk/lib/_internal/compiler/implementation/universe/universe.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/universe/universe.dart	(revision 20274)
+++ sdk/lib/_internal/compiler/implementation/universe/universe.dart	(working copy)
@@ -519,8 +519,8 @@
       return false;
     }
 
-    if (mask.isNullable && compiler.backend.isNullImplementation(other)) {
-      return appliesUntyped(element, compiler);
+    if (compiler.backend.isNullImplementation(other)) {
+      return mask.isNullable && appliesUntyped(element, compiler);
     } else if (mask.isExact) {
       return hasElementIn(self, element) && appliesUntyped(element, compiler);
     } else if (mask.isSubclass) {