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

runtime/bin/dartutils.h

Index: runtime/bin/dartutils.h
diff --git a/runtime/bin/dartutils.h b/runtime/bin/dartutils.h
index b416a9399cf76dd9c42d30db5dd060a7e763c4a1..9c3bc72c0e9811c8c58b2d0a6450319930b8007f 100644
--- a/runtime/bin/dartutils.h
+++ b/runtime/bin/dartutils.h
@@ -200,6 +200,7 @@ class DartUtils {
   static Dart_Handle ResolveUriInWorkingDirectory(Dart_Handle script_uri);
   static Dart_Handle FilePathFromUri(Dart_Handle script_uri);
   static Dart_Handle ResolveUri(Dart_Handle library_url, Dart_Handle url);
+  static Dart_Handle ResolveScript(Dart_Handle url);
 
   // Sniffs the specified text_buffer to see if it contains the magic number
   // representing a script snapshot. If the text_buffer is a script snapshot
@@ -231,9 +232,10 @@ class DartUtils {
 
   static const uint8_t magic_number[];
 
+  static Dart_Handle LibraryFilePath(Dart_Handle library_uri);
+
  private:
   static Dart_Handle SetWorkingDirectory();
-  static Dart_Handle LibraryFilePath(Dart_Handle library_uri);
   static Dart_Handle PrepareBuiltinLibrary(Dart_Handle builtin_lib,
                                            Dart_Handle internal_lib,
                                            bool is_service_isolate,