Fix casts and type assertions on mixins used in native classes

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of js_backend;
 
 class NativeEmitter {
 
   CodeEmitterTask emitter;
   CodeBuffer nativeBuffer;
@@ skipping 452 matching lines @@
       // The element type is a function type either directly or through
       // typedef(s).
       return false;
     }
 
     if (!element.isClass()) {
       compiler.cancel("Is check does not handle element", element: element);
       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 whcih case an interceptor must be
+    // satisfy a check against [element], in which 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 natibe instance that tries to spoof
+    // by a native class in case we get a native 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 (cls.isNative()) return true;
+    if (Elements.isNativeOrExtendsNative(cls)) return true;
     return isSupertypeOfNativeClass(element);
   }
 
   void assembleCode(CodeBuffer targetBuffer) {
     List<jsAst.Property> objectProperties = <jsAst.Property>[];
 
     void addProperty(String name, jsAst.Expression value) {
       objectProperties.add(new jsAst.Property(js.string(name), value));
     }
 
@@ skipping 23 matching lines @@
       if (emitter.compiler.enableMinification) targetBuffer.add(';');
       targetBuffer.add(jsAst.prettyPrint(
           new jsAst.ExpressionStatement(init), compiler));
       targetBuffer.add('\n');
     }
 
     targetBuffer.add(nativeBuffer);
     targetBuffer.add('\n');
   }
 }