Revert "Remove NullPointerException."

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";
   int get hashCode {
     try {
       return toInt();
@@ skipping 96 matching lines @@
   double floor() native "Double_floor";
   double ceil () native "Double_ceil";
   double truncate() native "Double_truncate";
   int toInt() native "Double_toInt";
   double toDouble() { return this; }
 
   double pow(num exponent) {
     if (exponent == 0) {
       return 1.0;  // ECMA-262 15.8.2.13
     }
-    if (exponent is! num) {
-      throw new ArgumentError(null);
-    }
+    // Throw NullPointerException if exponent is null.
     double doubleExponent = exponent.toDouble();
     if (isNaN || exponent.isNaN) {
       return double.NAN;
     }
     return _pow(doubleExponent);
   }
   double _pow(double exponent) native "Double_pow";
 
   String toStringAsFixed(int fractionDigits) {
     // See ECMAScript-262, 15.7.4.5 for details.
@@ skipping 93 matching lines @@
         return EQUAL;
       }
     } else if (isNaN) {
       return other.isNaN ? EQUAL : GREATER;
     } else {
       // Other is NaN.
       return LESS;
     }
   }
 }