Don't hide interceptors in mirrors.

sdk/lib/_internal/lib/js_number.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 _interceptors;
 
 /**
  * The super interceptor class for [JSInt] and [JSDouble]. The compiler
  * recognizes this class as an interceptor, and changes references to
  * [:this:] to actually use the receiver of the method, which is
@@ skipping 291 matching lines @@
 
   bool operator <=(num other) {
     if (other is !num) throw new ArgumentError(other);
     return JS('bool', '# <= #', this, other);
   }
 
   bool operator >=(num other) {
     if (other is !num) throw new ArgumentError(other);
     return JS('bool', '# >= #', this, other);
   }
+
+  Type get runtimeType => num;
 }
 
 /**
  * The interceptor class for [int]s.
  *
  * This class implements double since in JavaScript all numbers are doubles, so
  * while we want to treat `2.0` as an integer for some operations, its
  * interceptor should answer `true` to `is double`.
  */
 class JSInt extends JSNumber implements int, double {
@@ skipping 68 matching lines @@
 }
 
 class JSDouble extends JSNumber implements double {
   const JSDouble();
   Type get runtimeType => double;
 }
 
 class JSPositiveInt extends JSInt {}
 class JSUInt32 extends JSPositiveInt {}
 class JSUInt31 extends JSUInt32 {}