Fix off-by-one error in AstTyper, part 2.

src/typing.h

Index: src/typing.h
diff --git a/src/typing.h b/src/typing.h
index 6298e5a41b3d6824f45cc6879e408f48014ce9b0..a111ae5fd1594d14868ba2db80df27a34e2caf1e 100644
--- a/src/typing.h
+++ b/src/typing.h
@@ -83,8 +83,11 @@ class AstTyper: public AstVisitor {
   void ExitEffects() { store_ = store_.Pop(); }
 
   int variable_index(Variable* var) {
+    // Stack locals have the range [0 .. l]
+    // Parameters have the range [-1 .. p]
+    // We map this to [-p-2 .. -1, 0 .. l]
     return var->IsStackLocal() ? var->index() :
-           var->IsParameter() ? -var->index() - 1 : kNoVar;
+           var->IsParameter() ? -var->index() - 2 : kNoVar;
   }
 
   void VisitDeclarations(ZoneList<Declaration*>* declarations);