Introduce a lazy-compile stub for functions.

runtime/vm/gc_marker.cc

Index: runtime/vm/gc_marker.cc
===================================================================
--- runtime/vm/gc_marker.cc	(revision 34118)
+++ runtime/vm/gc_marker.cc	(working copy)
@@ -249,7 +249,12 @@
         // If the code wasn't strongly visited through other references
         // after skipping the function's code pointer, then we disconnect the
         // code from the function.
-        func->ptr()->code_ = Code::null();
+        bool is_closure =
+            Function::KindBits::decode(func->ptr()->kind_tag_) ==
+            RawFunction::kClosureFunction;
+        func->ptr()->code_ = is_closure
+            ? StubCode::LazyCompileClosure_entry()->code()
+            : Code::null();
         func->ptr()->unoptimized_code_ = Code::null();
         if (FLAG_log_code_drop) {
           // NOTE: This code runs while GC is in progress and runs within