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..37fd806091b1ccc3a69a5f282b7a9fdfaa7d5d23 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.
@@ -517,9 +518,10 @@ Dart_Handle Loader::ReloadNativeExtensions() {
 }
 
 
-Dart_Handle Loader::LoadUrlContents(Dart_Handle url,
-                                    uint8_t** payload,
-                                    intptr_t* payload_length) {
+Dart_Handle Loader::SendAndProcessReply(Dart_LibraryTag tag,
+                                        Dart_Handle url,
+                                        uint8_t** payload,
+                                        intptr_t* payload_length) {
   IsolateData* isolate_data =
       reinterpret_cast<IsolateData*>(Dart_CurrentIsolateData());
   ASSERT(isolate_data != NULL);
@@ -534,10 +536,10 @@ Dart_Handle Loader::LoadUrlContents(Dart_Handle url,
   ASSERT(isolate_data->HasLoader());
 
   // Now send a load request to the service isolate.
-  loader->SendRequest(Dart_kScriptTag, url, Dart_Null());
+  loader->SendRequest(tag, 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
@@ -557,6 +559,23 @@ Dart_Handle Loader::LoadUrlContents(Dart_Handle url,
 }
 
 
+Dart_Handle Loader::LoadUrlContents(Dart_Handle url,
+                                    uint8_t** payload,
+                                    intptr_t* payload_length) {
+  return SendAndProcessReply(Dart_kScriptTag, url, payload, payload_length);
+}
+
+
+Dart_Handle Loader::ResolveAsFilePath(Dart_Handle url,
+                                      uint8_t** payload,
+                                      intptr_t* payload_length) {
+  // TODO(28863) The use of tags that aren't declared in dart_api.h's enum

[Cutch, 2017/02/23 19:09:58]

I don't understand the issue here? These tags are being sent to the loader
isolate and not to the tag handler.

[rmacnak, 2017/02/23 19:27:41]

The cast here invokes undefined behavior because _Dart_kResolveAsFilePath is not
the value of any member of Dart_LibraryTag.

Making another attempt, I've managed to update several uses of Dart_LibraryTag
here to intptr_t without upsetting the signature of the actual tag handler. But
more changes are probably required here to get a clean UBSan build.

+  // is undefined behavior.
+  Dart_LibraryTag tag = static_cast<Dart_LibraryTag>(_Dart_kResolveAsFilePath);
+  return SendAndProcessReply(tag, url, payload, payload_length);
+}
+
+
 Dart_Handle Loader::LibraryTagHandler(Dart_LibraryTag tag,
                                       Dart_Handle library,
                                       Dart_Handle url) {