DBC: Fix reading locals out of a deopt frame.

runtime/vm/stack_frame_dbc.h

Index: runtime/vm/stack_frame_dbc.h
diff --git a/runtime/vm/stack_frame_dbc.h b/runtime/vm/stack_frame_dbc.h
index a59a417ed883b3b341be303dba46731e3669c626..84839f2bc32d30bb64b0d53f899ace9f46097622 100644
--- a/runtime/vm/stack_frame_dbc.h
+++ b/runtime/vm/stack_frame_dbc.h
@@ -10,7 +10,7 @@ namespace dart {
 /* DBC Frame Layout
 
 IMPORTANT: On DBC stack is growing upwards which is different from all other
-architectures. This enables effecient addressing for locals via unsigned index.
+architectures. This enables efficient addressing for locals via unsigned index.
 
                |                    | <- TOS
 Callee frame   | ...                |
@@ -65,6 +65,18 @@ DART_FORCE_INLINE static uword LocalVarAddress(uword fp, intptr_t index) {
   }
 }
 
+
+DART_FORCE_INLINE static intptr_t LocalVarIndex(intptr_t fp_offset,
+                                                intptr_t var_index) {
+  ASSERT(var_index != 0);
+  if (var_index > 0) {
+    return fp_offset - var_index;
+  } else {
+    return fp_offset - (var_index + 1);
+  }
+}
+
+
 DART_FORCE_INLINE static uword ParamAddress(uword fp, intptr_t reverse_index) {
   return fp - (kDartFrameFixedSize + reverse_index) * kWordSize;
 }