Make Bigint instances immutable by removing all setters.

runtime/lib/integers.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.
 
 // TODO(srdjan): fix limitations.
 // - shift amount must be a Smi.
 class _IntegerImplementation extends _Num {
   // The Dart class _Bigint extending _IntegerImplementation requires a
   // default constructor.
 
@@ skipping 247 matching lines @@
     var mask = radix - 1;
     do {
       string._setAt(--length, _digits.codeUnitAt(value & mask));
       value >>= bitsPerDigit;
     } while (value > 0);
     return string;
   }
 
   _leftShiftWithMask32(count, mask)  native "Integer_leftShiftWithMask32";
 
-  // TODO(regis): Make this method private once the plumbing to invoke it from
-  // dart:math is in place. Move the argument checking to dart:math.
   // Return pow(this, e) % m.
   int modPow(int e, int m) {
-    if (e is! int) throw new ArgumentError(e);
-    if (m is! int) throw new ArgumentError(m);
+    if (e is! int || e < 0) throw new ArgumentError(e);
+    if (m is! int || m <= 0) throw new ArgumentError(m);
     if (e is _Bigint || m is _Bigint) {
       return _toBigint().modPow(e, m);
     }
     if (e < 1) return 1;
     int b = this;
     if (b < 0 || b > m) {
       b = b % m;
     }
     int r = 1;
     while (e > 0) {
@@ skipping 219 matching lines @@
   // Shift by mint exceeds range that can be handled by the VM.
   int _shrFromInt(int other) {
     if (other < 0) {
       return -1;
     } else {
       return 0;
     }
   }
   int _shlFromInt(int other) native "Mint_shlFromInt";
 }