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

runtime/bin/builtin.h

Index: runtime/bin/builtin.h
diff --git a/runtime/bin/builtin.h b/runtime/bin/builtin.h
index a4893d9bf7fc1541c6153952dc2f1f882b646d06..5029cf1b30c2350b8f8ef866c3da5fe7ad3e1693 100644
--- a/runtime/bin/builtin.h
+++ b/runtime/bin/builtin.h
@@ -51,6 +51,11 @@ class Builtin {
 
   static Dart_Handle SetLoadPort(Dart_Port port);
 
+  static Dart_Port LoadPort() {
+    ASSERT(load_port_ != ILLEGAL_PORT);
+    return load_port_;
+  }
+
  private:
   // Map specified URI to an actual file name from 'source_paths' and read
   // the file.