[vm] Revise Dart_IntegerToHexCString to avoid dependency on Bigint

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index ed2c2c13ce9c63bf2b246838ef52332f122e69b2..6e20a1bc57dcd9e2d651ed45dbfe015f7ba29b19 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -6458,6 +6458,11 @@ class Integer : public Number {
   // Returns 0, -1 or 1.
   virtual int CompareWith(const Integer& other) const;
 
+  // Converts integer to hex string.
+  // TODO(alexmarkov): this method can become non-virtual once Bigint class is
+  // decoupled from Integer hierarchy.
+  virtual const char* ToHexCString(Zone* zone) const;
+
   // Return the most compact presentation of an integer.
   RawInteger* AsValidInteger() const;
 
@@ -6615,6 +6620,8 @@ class Bigint : public Integer {
 
   virtual int CompareWith(const Integer& other) const;
 
+  virtual const char* ToHexCString(Zone* zone) const;
+
   virtual bool CheckAndCanonicalizeFields(Thread* thread,
                                           const char** error_str) const;
 
@@ -6643,7 +6650,6 @@ class Bigint : public Integer {
   uint32_t DigitAt(intptr_t index) const;
 
   const char* ToDecCString(Zone* zone) const;
-  const char* ToHexCString(Zone* zone) const;
 
   static const intptr_t kBitsPerDigit = 32;  // Same as _Bigint._DIGIT_BITS
   static const intptr_t kBytesPerDigit = 4;