Don't hide interceptors in mirrors.

tests/compiler/dart2js/value_range_test.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.
 
 import "package:expect/expect.dart";
 import 'compiler_helper.dart';
 
 const int REMOVED = 0;
 const int ABOVE_ZERO = 1;
 const int BELOW_LENGTH = 2;
@@ skipping 310 matching lines @@
     operator <=(other) {}
     operator >=(other) {}
     operator ==(other) {}
   }
   class JSInt extends JSNumber {
     operator~() => this;
   }
   class JSDouble extends JSNumber {
   }
   class JSNull {
+    bool operator==(other) => identical(null, other);
+    get hashCode => throw "JSNull.hashCode not implemented.";
+    String toString() => 'Null';
+    Type get runtimeType => Null;
+    noSuchMethod(x) => super.noSuchMethod(x);
   }
   class JSBool {
   }
   class JSFunction {
   }
   class ObjectInterceptor {
   }
   class JSPositiveInt extends JSInt {}
   class JSUInt32 extends JSPositiveInt {}
   class JSUInt31 extends JSUInt32 {}
@@ skipping 42 matching lines @@
       break;
   }
 }
 
 
 main() {
   for (int i = 0; i < TESTS.length;  i += 2) {
     expect(TESTS[i], TESTS[i + 1]);
   }
 }