Make Bigint instances immutable by removing all setters.

runtime/vm/intrinsifier_arm64.cc

Index: runtime/vm/intrinsifier_arm64.cc
===================================================================
--- runtime/vm/intrinsifier_arm64.cc	(revision 43422)
+++ runtime/vm/intrinsifier_arm64.cc	(working copy)
@@ -688,30 +688,6 @@
 }
 
 
-void Intrinsifier::Bigint_setNeg(Assembler* assembler) {
-  __ ldr(R0, Address(SP, 0 * kWordSize));
-  __ ldr(R1, Address(SP, 1 * kWordSize));
-  __ StoreIntoObject(R1, FieldAddress(R1, Bigint::neg_offset()), R0, false);
-  __ ret();
-}
-
-
-void Intrinsifier::Bigint_setUsed(Assembler* assembler) {
-  __ ldr(R0, Address(SP, 0 * kWordSize));
-  __ ldr(R1, Address(SP, 1 * kWordSize));
-  __ StoreIntoObject(R1, FieldAddress(R1, Bigint::used_offset()), R0);
-  __ ret();
-}
-
-
-void Intrinsifier::Bigint_setDigits(Assembler* assembler) {
-  __ ldr(R0, Address(SP, 0 * kWordSize));
-  __ ldr(R1, Address(SP, 1 * kWordSize));
-  __ StoreIntoObject(R1, FieldAddress(R1, Bigint::digits_offset()), R0, false);
-  __ ret();
-}
-
-
 void Intrinsifier::Bigint_absAdd(Assembler* assembler) {
   // static void _absAdd(Uint32List digits, int used,
   //                     Uint32List a_digits, int a_used,