Make Bigint instances immutable by removing all setters.

runtime/vm/bootstrap_natives.h

 // 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.
 
 #ifndef VM_BOOTSTRAP_NATIVES_H_
 #define VM_BOOTSTRAP_NATIVES_H_
 
 #include "vm/native_entry.h"
 
 // bootstrap dart natives used in the core dart library.
@@ skipping 40 matching lines @@
   V(SendPortImpl_get_hashcode, 1)                                              \
   V(SendPortImpl_sendInternal_, 2)                                             \
   V(Smi_shlFromInt, 2)                                                         \
   V(Smi_shrFromInt, 2)                                                         \
   V(Smi_bitNegate, 1)                                                          \
   V(Smi_bitLength, 1)                                                          \
   V(Mint_bitNegate, 1)                                                         \
   V(Mint_bitLength, 1)                                                         \
   V(Mint_shlFromInt, 2)                                                        \
   V(Bigint_getNeg, 1)                                                          \
-  V(Bigint_setNeg, 2)                                                          \
   V(Bigint_getUsed, 1)                                                         \
-  V(Bigint_setUsed, 2)                                                         \
   V(Bigint_getDigits, 1)                                                       \
-  V(Bigint_setDigits, 2)                                                       \
-  V(Bigint_allocate, 1)                                                        \
+  V(Bigint_allocate, 4)                                                        \
   V(Double_getIsNegative, 1)                                                   \
   V(Double_getIsInfinite, 1)                                                   \
   V(Double_getIsNaN, 1)                                                        \
   V(Double_add, 2)                                                             \
   V(Double_sub, 2)                                                             \
   V(Double_mul, 2)                                                             \
   V(Double_div, 2)                                                             \
   V(Double_trunc_div, 2)                                                       \
   V(Double_remainder, 2)                                                       \
   V(Double_modulo, 2)                                                          \
@@ skipping 316 matching lines @@
   static void DN_##name(Dart_NativeArguments args);
 
   BOOTSTRAP_NATIVE_LIST(DECLARE_BOOTSTRAP_NATIVE)
 
 #undef DECLARE_BOOTSTRAP_NATIVE
 };
 
 }  // namespace dart
 
 #endif  // VM_BOOTSTRAP_NATIVES_H_