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

runtime/vm/assembler.h

Index: runtime/vm/assembler.h
diff --git a/runtime/vm/assembler.h b/runtime/vm/assembler.h
index d8249ba4377fb4dc2645bd26d8c37a9ecfc8dde5..6ef8ae61c0af640748e2c8f3de6aeb2c4033523c 100644
--- a/runtime/vm/assembler.h
+++ b/runtime/vm/assembler.h
@@ -89,6 +89,11 @@ class AssemblerBuffer : public ValueObject {
     cursor_ -= sizeof(T);
   }
 
+  // Return address to code at |position| bytes.
+  uword Address(intptr_t position) {
+    return reinterpret_cast<uword>(contents_ + position);
+  }
+
   template<typename T> T Load(intptr_t position) {
     ASSERT(position >= 0 &&
            position <= (Size() - static_cast<intptr_t>(sizeof(T))));