Fixes a couple problems with GC of unoptimized code.

runtime/vm/object.cc

Index: runtime/vm/object.cc
===================================================================
--- runtime/vm/object.cc	(revision 30197)
+++ runtime/vm/object.cc	(working copy)
@@ -4111,6 +4111,7 @@
 void Function::DetachCode() const {
   // Set unoptimized code as non-entrant, and set code and unoptimized code
   // to null.
+  ASSERT(!HasOptimizedCode());
   CodePatcher::PatchEntry(Code::Handle(unoptimized_code()));
   StorePointer(&raw_ptr()->code_, Code::null());
   StorePointer(&raw_ptr()->unoptimized_code_, Code::null());
@@ -4118,6 +4119,10 @@
 
 
 void Function::ReattachCode(const Code& code) const {
+  ASSERT(CurrentCode() == Code::null());
+  ASSERT(unoptimized_code() == Code::null());
+  ASSERT(!code.IsNull());
+  ASSERT(!code.is_optimized());
   StorePointer(&raw_ptr()->code_, code.raw());
   StorePointer(&raw_ptr()->unoptimized_code_, code.raw());
   CodePatcher::RestoreEntry(code);