Precompilation: The root library may have no survivors if it just re-exports main.

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 1620 matching lines @@
     lib = cls.library();
     name = cls.DictionaryName();
     lib.RemoveObject(cls, name);
   }
 }
 
 
 void Precompiler::DropLibraries() {
   const GrowableObjectArray& retained_libraries =
       GrowableObjectArray::Handle(Z, GrowableObjectArray::New());
+  Library& root_lib = Library::Handle(Z, I->object_store()->root_library());
   Library& lib = Library::Handle(Z);
 
   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++;
     }
-    bool retain = (entries > 0) || lib.is_dart_scheme();
+    // 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());
     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 1081 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