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

runtime/vm/assembler_ia32.h

Index: runtime/vm/assembler_ia32.h
diff --git a/runtime/vm/assembler_ia32.h b/runtime/vm/assembler_ia32.h
index e00528a506a6540e9152894161e7ce231bf9a494..a27481e787ce7de3b2c9b69561bfbef9233e0b3f 100644
--- a/runtime/vm/assembler_ia32.h
+++ b/runtime/vm/assembler_ia32.h
@@ -789,6 +789,11 @@ class Assembler : public ValueObject {
   void Bind(Label* label);
   void Jump(Label* label) { jmp(label); }
 
+  // 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_; }