Make mixins work on native classes.

sdk/lib/_internal/compiler/implementation/ssa/optimize.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 ssa;
 
 abstract class OptimizationPhase {
   String get name;
   void visitGraph(HGraph graph);
 }
@@ skipping 797 matching lines @@
       return false;
     }
 
     // [interceptedClasses] is sparse - it is just the classes that define some
     // intercepted method.  Their subclasses (that inherit the method) are
     // implicit, so we have to extend them.
 
     TypeMask receiverMask = receiverType.computeMask(compiler);
     return interceptedClasses
         .where((cls) => cls != compiler.objectClass)
-        .map((cls) => new TypeMask.subclass(cls.rawType))
+        .map((cls) => backend.classesMixedIntoNativeClasses.contains(cls)
+            ? new TypeMask.subtype(cls.rawType)
+            : new TypeMask.subclass(cls.rawType))
         .every((mask) => receiverMask.intersection(mask, compiler).isEmpty);
   }
 
   HInstruction tryComputeConstantInterceptor(HInstruction input,
                                              Set<ClassElement> intercepted) {
     if (input == graph.explicitReceiverParameter) {
       // If `explicitReceiverParameter` is set it means the current method is an
       // interceptor method, and `this` is the interceptor.  The caller just did
       // `getInterceptor(foo).currentMethod(foo)` to enter the current method.
       return graph.thisInstruction;
@@ skipping 857 matching lines @@
     HBasicBlock block = user.block;
     block.addAfter(user, interceptor);
     block.rewrite(user, interceptor);
     block.remove(user);
 
     // The interceptor will be removed in the dead code elimination
     // phase. Note that removing it here would not work because of how
     // the [visitBasicBlock] is implemented.
   }
 }