Revert "Fix casts and type assertions on mixins used in native classes."

sdk/lib/_internal/compiler/implementation/js_backend/native_emitter.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/native_emitter.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/native_emitter.dart b/sdk/lib/_internal/compiler/implementation/js_backend/native_emitter.dart
index 13cc42b9cab04eb484b4db8e810383084580cb5f..c5ccf96c8863fd632c0815648eef71cf74425733 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/native_emitter.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/native_emitter.dart
@@ -464,21 +464,19 @@ class NativeEmitter {
       return false;
     }
 
-    if (backend.classesMixedIntoNativeClasses.contains(element)) return true;
-
     return subtypes[element] != null;
   }
 
   bool requiresNativeIsCheck(Element element) {
     // TODO(sra): Remove this function.  It determines if a native type may
-    // satisfy a check against [element], in which case an interceptor must be
+    // satisfy a check against [element], in whcih case an interceptor must be
     // used.  We should also use an interceptor if the check can't be satisfied
-    // by a native class in case we get a native instance that tries to spoof
+    // by a native class in case we get a natibe instance that tries to spoof
     // the type info.  i.e the criteria for whether or not to use an interceptor
     // is whether the receiver can be native, not the type of the test.
     if (!element.isClass()) return false;
     ClassElement cls = element;
-    if (Elements.isNativeOrExtendsNative(cls)) return true;
+    if (cls.isNative()) return true;
     return isSupertypeOfNativeClass(element);
   }