Correctly set root_library based on the application script URI instead of looking for the library t…

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 2a7d3247eb75c0dd30fa5ecc24c838c3182481a1..d63328a65c83235a2341af07e203733898738de0 100644
--- a/runtime/bin/gen_snapshot.cc
+++ b/runtime/bin/gen_snapshot.cc
@@ -1749,8 +1749,12 @@ int main(int argc, char** argv) {
         ParseEntryPointsManifestIfPresent();
 
     if (kernel_program != NULL) {
-      Dart_Handle library = Dart_LoadKernel(kernel_program);
-      if (Dart_IsError(library)) FATAL("Failed to load app from Kernel IR");
+      Dart_Handle resolved_uri = ResolveUriInWorkingDirectory(app_script_name);
+      CHECK_RESULT(resolved_uri);
+      Dart_Handle library =
+          Dart_LoadScript(resolved_uri, Dart_Null(),
+                          reinterpret_cast<Dart_Handle>(kernel_program), 0, 0);
+      CHECK_RESULT(library);
     } else {
       // Set up the library tag handler in such a manner that it will use the
       // URL mapping specified on the command line to load the libraries.