Fixes a couple problems with GC of unoptimized code.

runtime/vm/object.h

Index: runtime/vm/object.h
===================================================================
--- runtime/vm/object.h	(revision 30598)
+++ runtime/vm/object.h	(working copy)
@@ -1508,13 +1508,6 @@
   // Sets function's code and code's function.
   void SetCode(const Code& value) const;
 
-  // Detaches code from the function by setting the code to null, and patches
-  // the code to be non-entrant.
-  void DetachCode() const;
-
-  // Reattaches code to the function, and patches the code to be entrant.
-  void ReattachCode(const Code& code) const;
-
   // Disables optimized code and switches to unoptimized code.
   void SwitchToUnoptimizedCode() const;
 
@@ -2002,6 +1995,9 @@
 
   FINAL_HEAP_OBJECT_IMPLEMENTATION(Function, Object);
   friend class Class;
+  // RawFunction::VisitFunctionPointers accesses the private constructor of
+  // Function.
+  friend class RawFunction;
 };
 
 
@@ -3360,6 +3356,10 @@
 
   FINAL_HEAP_OBJECT_IMPLEMENTATION(Code, Object);
   friend class Class;
+
+  // So that the RawFunction pointer visitor can determine whether code the
+  // function points to is optimized.
+  friend class RawFunction;
 };
 
 
@@ -5020,6 +5020,8 @@
   friend class TwoByteString;
   friend class ExternalOneByteString;
   friend class ExternalTwoByteString;
+  // So that the MarkingVisitor can print a debug string from a NoHandleScope.
+  friend class MarkingVisitor;
 };