Fix GenerateNegativeLookup to work with non-symbols as a dictionary key.

src/ia32/stub-cache-ia32.cc

Index: src/ia32/stub-cache-ia32.cc
===================================================================
--- src/ia32/stub-cache-ia32.cc	(revision 5070)
+++ src/ia32/stub-cache-ia32.cc	(working copy)
@@ -184,6 +184,12 @@
       // Stop if found the property.
       __ cmp(entity_name, Handle<String>(name));
       __ j(equal, miss_label, not_taken);
+
+      // Check if the entry name is not a symbol.
+      __ mov(entity_name, FieldOperand(entity_name, HeapObject::kMapOffset));
+      __ test_b(FieldOperand(entity_name, Map::kInstanceTypeOffset),
+                kIsSymbolMask);
+      __ j(zero, miss_label, not_taken);
     } else {
       // Give up probing if still not found the undefined value.
       __ j(not_equal, miss_label, not_taken);