Add option to gen_snapshot for creating a Makefile describing a snapshot's dependencies.

runtime/bin/loader.cc

Index: runtime/bin/loader.cc
diff --git a/runtime/bin/loader.cc b/runtime/bin/loader.cc
index 70ee44471d6628f2a7a1892515fe274d6a771c2b..f83081d71e91b0ba8b80497fd5eb68940306412f 100644
--- a/runtime/bin/loader.cc
+++ b/runtime/bin/loader.cc
@@ -20,6 +20,7 @@ namespace bin {
 static bool trace_loader = false;
 // Keep in sync with loader.dart.
 static const intptr_t _Dart_kImportExtension = 9;
+static const intptr_t _Dart_kResolveAsFilePath = 10;
 
 Loader::Loader(IsolateData* isolate_data)
     : port_(ILLEGAL_PORT),
@@ -424,7 +425,7 @@ bool Loader::ProcessResultLocked(Loader* loader, Loader::IOResult* result) {
 }
 
 
-bool Loader::ProcessUrlLoadResultLocked(Loader* loader,
+bool Loader::ProcessPayloadResultLocked(Loader* loader,
                                         Loader::IOResult* result) {
   // A negative result tag indicates a loading error occurred in the service
   // isolate. The payload is a C string of the error message.
@@ -537,7 +538,50 @@ Dart_Handle Loader::LoadUrlContents(Dart_Handle url,
   loader->SendRequest(Dart_kScriptTag, url, Dart_Null());
 
   // Wait for a reply to the load request.
-  loader->BlockUntilComplete(ProcessUrlLoadResultLocked);
+  loader->BlockUntilComplete(ProcessPayloadResultLocked);
+
+  // Copy fields from the loader before deleting it.
+  // The payload array itself which was malloced above is freed by
+  // the caller of LoadUrlContents.
+  Dart_Handle error = loader->error();
+  *payload = loader->payload_;
+  *payload_length = loader->payload_length_;
+
+  // Destroy the loader. The destructor does a bunch of leg work.
+  delete loader;
+
+  // An error occurred during loading.
+  if (!Dart_IsNull(error)) {
+    return error;
+  }
+  return Dart_Null();
+}
+
+
+Dart_Handle Loader::ResolveAsFilePath(Dart_Handle url,
+                                      uint8_t** payload,
+                                      intptr_t* payload_length) {
+  IsolateData* isolate_data =
+      reinterpret_cast<IsolateData*>(Dart_CurrentIsolateData());
+  ASSERT(isolate_data != NULL);
+  ASSERT(!isolate_data->HasLoader());
+  Loader* loader = NULL;
+
+  // Setup the loader. The constructor does a bunch of leg work.
+  loader = new Loader(isolate_data);
+  loader->Init(isolate_data->package_root, isolate_data->packages_file,
+               DartUtils::original_working_directory, NULL);
+  ASSERT(loader != NULL);
+  ASSERT(isolate_data->HasLoader());
+
+  // Now send a load request to the service isolate.
+  // TODO(28863) The use of tags that aren't declared in dart_api.h's enum
+  // is undefined behavior.
+  Dart_LibraryTag tag = static_cast<Dart_LibraryTag>(_Dart_kResolveAsFilePath);
+  loader->SendRequest(tag, url, Dart_Null());
+
+  // Wait for a reply to the load request.
+  loader->BlockUntilComplete(ProcessPayloadResultLocked);
 
   // Copy fields from the loader before deleting it.
   // The payload array itself which was malloced above is freed by