Rework standalone to use a synchronous loader that does not invoke Dart code

runtime/bin/gen_snapshot.cc

Index: runtime/bin/gen_snapshot.cc
diff --git a/runtime/bin/gen_snapshot.cc b/runtime/bin/gen_snapshot.cc
index 3e182486d25c3cee6403f445a13b2503207bfc76..b092c2b25a0007c08ce9a83a58b33617d0d9206f 100644
--- a/runtime/bin/gen_snapshot.cc
+++ b/runtime/bin/gen_snapshot.cc
@@ -598,6 +598,9 @@ static Dart_Handle LoadGenericSnapshotCreationScript(
   // Load the builtin library to make it available in the snapshot
   // for importing.
   lib = Builtin::LoadAndCheckLibrary(id);
+  if (Dart_IsError(lib)) {
+    fprintf(stderr, "%s", Dart_GetError(lib));

[turnidge, 2016/06/03 17:50:25]

Debug stuff....

[Cutch, 2016/06/03 22:07:32]

Done.

+  }
   ASSERT(!Dart_IsError(lib));
   return lib;
 }