Add documentation to dart:math pow function.

runtime/lib/math_patch.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 "dart:typed_data";
 
 // A VM patch of the dart:math library.
 
 // If [x] is an [int] and [exponent] is a non-negative [int], the result is
 // an [int], otherwise the result is a [double].
 patch num pow(num x, num exponent) {
   if ((x is int) && (exponent is int) && (exponent >= 0)) {
     return _intPow(x, exponent);
   }
   // doublePow will call exponent.toDouble().
   return _doublePow(x.toDouble(), exponent);
 }
 
 double _doublePow(double base, num exponent) {
   if (exponent == 0) {
     return 1.0;  // ECMA-262 15.8.2.13
   }
-  if (exponent is! num) {
-    throw new ArgumentError(null);
-  }
+  if (base == 1.0) return 1.0;
   double doubleExponent = exponent.toDouble();
   if (base.isNaN || exponent.isNaN) {
     return double.NAN;
   }
   return _pow(base, doubleExponent);
 }
 
 double _pow(double base, double exponent) native "Math_doublePow";
 
 int _intPow(int base, int exponent) {
@@ skipping 127 matching lines @@
   static int _nextSeed() {
     if (_prng == null) {
       // TODO(iposva): Use system to get a random seed.
       _prng = new Random(new DateTime.now().millisecondsSinceEpoch);
     }
     // Trigger the PRNG once to change the internal state.
     _prng._nextState();
     return _prng._state[kSTATE_LO];
   }
 }