Only reload libraries when they may have been modified.

runtime/bin/builtin.cc

Index: runtime/bin/builtin.cc
diff --git a/runtime/bin/builtin.cc b/runtime/bin/builtin.cc
index 04fb0600e8742451bd48a7f567c93a52e81313cf..2c303a17d2350b18a148ad9a4aaac6b7008fbc63 100644
--- a/runtime/bin/builtin.cc
+++ b/runtime/bin/builtin.cc
@@ -119,7 +119,7 @@ Dart_Handle Builtin::LoadLibrary(Dart_Handle url, BuiltinLibraryId id) {
   ASSERT(static_cast<int>(id) >= 0);
   ASSERT(static_cast<int>(id) < num_libs_);
 
-  Dart_Handle library = Dart_LoadLibrary(url, Source(id), 0, 0);
+  Dart_Handle library = Dart_LoadLibrary(url, Dart_Null(), Source(id), 0, 0);
   if (!Dart_IsError(library) && (builtin_libraries_[id].has_natives_)) {
     // Setup the native resolver for built in library functions.
     DART_CHECK_VALID(