Revert "[vm] Revise Dart_IntegerToHexCString to avoid dependency on Bigint"

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 5e43e251d91229679752d35af80908e1fca089e8..2cac9f950c190b2a57177b881cffb6db21bb2538 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -18241,16 +18241,6 @@ RawInteger* Integer::AsValidInteger() const {
   return raw();
 }
 
-const char* Integer::ToHexCString(Zone* zone) const {
-  ASSERT(IsSmi() || IsMint());  // Bigint has its own implementation.
-  int64_t value = AsInt64Value();
-  if (value < 0) {
-    return OS::SCreate(zone, "-0x%" PRIX64, static_cast<uint64_t>(-value));
-  } else {
-    return OS::SCreate(zone, "0x%" PRIX64, static_cast<uint64_t>(value));
-  }
-}
-
 RawInteger* Integer::ArithmeticOp(Token::Kind operation,
                                   const Integer& other,
                                   Heap::Space space) const {