Fixes a couple problems with GC of unoptimized code.

runtime/vm/pages.h

Index: runtime/vm/pages.h
===================================================================
--- runtime/vm/pages.h	(revision 30598)
+++ runtime/vm/pages.h	(working copy)
@@ -13,6 +13,7 @@
 
 DECLARE_FLAG(bool, collect_code);
 DECLARE_FLAG(bool, log_code_drop);
+DECLARE_FLAG(bool, always_drop_code);
 
 // Forward declarations.
 class Heap;
@@ -199,9 +200,9 @@
   RawObject* FindObject(FindObjectVisitor* visitor,
                         HeapPage::PageType type) const;
 
-  // Runs a visitor that attempts to drop references to code that has not
-  // been run in awhile.
-  void TryDetachingCode();
+  // Checks if enough time has elapsed since the last attempt to collect
+  // code.
+  bool ShouldCollectCode();
 
   // Collect the garbage in the page space using mark-sweep.
   void MarkSweep(bool invoke_api_callbacks);