Last reference to a library may be through a top-level type.

runtime/vm/precompiler.cc

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/precompiler.h"
 
 #include "vm/aot_optimizer.h"
 #include "vm/assembler.h"
 #include "vm/ast_printer.h"
 #include "vm/branch_optimizer.h"
@@ skipping 1702 matching lines @@
 
   for (intptr_t i = 0; i < libraries_.Length(); i++) {
     lib ^= libraries_.At(i);
     lib.DropDependencies();
     intptr_t entries = 0;
     DictionaryIterator it(lib);
     while (it.HasNext()) {
       it.GetNext();
       entries++;
     }
-    // The root library might have no surviving members if it only exports main
-    // from another library. It will still be referenced from the object store,
-    // so retain it.
-    bool retain = (entries > 0) ||
-                  lib.is_dart_scheme() ||
-                  (lib.raw() == root_lib.raw());
+    bool retain = false;
+    if (entries > 0) {
+      retain = true;
+    } else if (lib.is_dart_scheme()) {
+      // The core libraries are referenced from the object store.
+      retain = true;
+    } else if (lib.raw() == root_lib.raw()) {
+      // The root library might have no surviving members if it only exports
+      // main from another library. It will still be referenced from the object
+      // store, so retain it.
+      retain = true;
+    } else {
+      // A type for a top-level class may be referenced from an object pool as
+      // part of an error message.
+      const Class& top = Class::Handle(Z, lib.toplevel_class());
+      if (classes_to_retain_.Lookup(&top) != NULL) {
+        retain = true;
+      }
+    }
+
     if (retain) {
       lib.set_index(retained_libraries.Length());
       retained_libraries.Add(lib);
     } else {
       dropped_library_count_++;
       lib.set_index(-1);
       if (FLAG_trace_precompiler) {
         THR_Print("Dropping library %s\n", lib.ToCString());
       }
     }
@@ skipping 1109 matching lines @@
       CompilationPipeline::New(thread->zone(), function);
 
   ASSERT(FLAG_precompiled_mode);
   const bool optimized = function.IsOptimizable();  // False for natives.
   return PrecompileFunctionHelper(pipeline, function, optimized);
 }
 
 #endif  // DART_PRECOMPILER
 
 }  // namespace dart