Fix slack tracking when instance prototype changes.

src/arm/stub-cache-arm.cc

Index: src/arm/stub-cache-arm.cc
diff --git a/src/arm/stub-cache-arm.cc b/src/arm/stub-cache-arm.cc
index d3b58624c8b24058eda3cdec1742bc7ccc36c739..9fc39d4ad8610d4687a758bd56734dd5a6173904 100644
--- a/src/arm/stub-cache-arm.cc
+++ b/src/arm/stub-cache-arm.cc
@@ -3467,7 +3467,13 @@ Handle<Code> ConstructStubCompiler::CompileConstructStub(
   // r1: constructor function
   // r2: initial map
   // r7: undefined
+  ASSERT(function->has_initial_map());
   __ ldrb(r3, FieldMemOperand(r2, Map::kInstanceSizeOffset));
+#ifdef DEBUG
+  int instance_size = function->initial_map()->instance_size();
+  __ cmp(r3, Operand(instance_size >> kPointerSizeLog2));
+  __ Check(eq, "Instance size of initial map changed.");
+#endif
   __ AllocateInNewSpace(r3, r4, r5, r6, &generic_stub_call, SIZE_IN_WORDS);
 
   // Allocated the JSObject, now initialize the fields. Map is set to initial
@@ -3525,7 +3531,6 @@ Handle<Code> ConstructStubCompiler::CompileConstructStub(
   }
 
   // Fill the unused in-object property fields with undefined.
-  ASSERT(function->has_initial_map());
   for (int i = shared->this_property_assignments_count();
        i < function->initial_map()->inobject_properties();
        i++) {