Local closures may be the only survivors in a library.

runtime/vm/precompiler.h

Index: runtime/vm/precompiler.h
diff --git a/runtime/vm/precompiler.h b/runtime/vm/precompiler.h
index bbefd2952b764d76d06be73f07474f514c940849..5b6c7ac5526148bcd538c477ee289efffaf524d5 100644
--- a/runtime/vm/precompiler.h
+++ b/runtime/vm/precompiler.h
@@ -309,6 +309,7 @@ class Precompiler : public ValueObject {
   void CheckForNewDynamicFunctions();
   void TraceConstFunctions();
 
+  void TraceForRetainedFunctions();
   void DropFunctions();
   void DropFields();
   void TraceTypesFromRetainedClasses();
@@ -362,6 +363,7 @@ class Precompiler : public ValueObject {
   SymbolSet sent_selectors_;
   FunctionSet enqueued_functions_;
   FieldSet fields_to_retain_;
+  FunctionSet functions_to_retain_;
   ClassSet classes_to_retain_;
   TypeArgumentsSet typeargs_to_retain_;
   AbstractTypeSet types_to_retain_;