Fix runtimeType for strings, integers and double: return their interface type.

runtime/lib/double.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.
 
 class _Double implements double {
   factory _Double.fromInteger(int value)
       native "Double_doubleFromInteger";
+
+  Type get runtimeType => double;
+
   int get _identityHashCode {
     if (isNaN || isInfinite) return 0;
     return toInt();
   }
   double operator +(num other) {
     return _add(other.toDouble());
   }
   double _add(double other) native "Double_add";
 
   double operator -(num other) {
@@ skipping 208 matching lines @@
         return EQUAL;
       }
     } else if (isNaN) {
       return other.isNaN ? EQUAL : GREATER;
     } else {
       // Other is NaN.
       return LESS;
     }
   }
 }