Track prologue offset of Code and use for more precise return addresses

runtime/vm/assembler_x64.h

Index: runtime/vm/assembler_x64.h
diff --git a/runtime/vm/assembler_x64.h b/runtime/vm/assembler_x64.h
index a257d770bcfab6cd7a53b0fd433ac64a0b039fa3..4bf6a573efca76738d757ab6683f71ae155d6910 100644
--- a/runtime/vm/assembler_x64.h
+++ b/runtime/vm/assembler_x64.h
@@ -885,6 +885,11 @@ class Assembler : public ValueObject {
 
   const Code::Comments& GetCodeComments() const;
 
+  // Address of code at offset.
+  uword CodeAddress(intptr_t offset) {
+    return buffer_.Address(offset);
+  }
+
   intptr_t CodeSize() const { return buffer_.Size(); }
   intptr_t prologue_offset() const { return prologue_offset_; }