File constructor understands lastModified.

Source/bindings/v8/custom/V8FileCustom.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 44 matching lines @@
     } else {
         const int sequenceArgumentIndex = 0;
         if (toV8Sequence(info[sequenceArgumentIndex], length, info.GetIsolate()).IsEmpty()) {
             throwTypeError(ExceptionMessages::failedToConstruct("File", ExceptionMessages::notAnArrayTypeArgumentOrValue(sequenceArgumentIndex + 1)), info.GetIsolate());
             return;
         }
     }
 
     V8TRYCATCH_FOR_V8STRINGRESOURCE_VOID(V8StringResource<>, fileName, info[1]);
 
-    String contentType;
-    String endings = "transparent"; // default if no BlobPropertyBag is passed
+    V8BlobCustomHelpers::ParsedProperties properties(true);
     if (info.Length() > 2) {
         if (!info[2]->IsObject()) {
             throwTypeError(ExceptionMessages::failedToConstruct("File", "The 3rd argument is not of type Object."), info.GetIsolate());
             return;
         }
 
-        if (!V8BlobCustomHelpers::processBlobPropertyBag(info[2], "File", contentType, endings, info.GetIsolate()))
+        if (!properties.parseBlobPropertyBag(info[2], "File", info.GetIsolate()))
             return;
+    } else {
+        properties.setDefaultLastModifiedDate();
     }
 
     BlobBuilder blobBuilder;
     v8::Local<v8::Object> blobParts = v8::Local<v8::Object>::Cast(info[0]);
-    if (!V8BlobCustomHelpers::processBlobParts(blobParts, length, endings, blobBuilder, info.GetIsolate()))
+    if (!V8BlobCustomHelpers::processBlobParts(blobParts, length, properties.endings, blobBuilder, info.GetIsolate()))
         return;
 
-    RefPtr<File> file = blobBuilder.createFile(contentType, fileName, currentTime());
+    RefPtr<File> file = blobBuilder.createFile(properties.contentType, fileName, properties.lastModifiedDate());
     v8SetReturnValue(info, file.release());
 }
 
+void V8File::lastModifiedDateAttributeGetterCustom(const v8::PropertyCallbackInfo<v8::Value>& info)
+{
+    // The auto-generated getters return null when the method in the underlying
+    // implementation returns NaN. Here, we need to return a Date whose value
+    // is NaN when the File constructor receives such a Date. WebIDL calls it
+    // an "undefined date". Fortunately, lastModifiedDate never needs to return
+    // null, so we can get away with mapping NaN to "new Date(NaN)" instead of
+    // "null".

[kinuko, 2013/11/18 12:21:43]

Ah sorry, here we could be given NaN when last modified date for the backing
file is not known, when the file is created not by constructor but for a real
file by the embedder or client, i.e. by chromium.  This was done purely for
internal implementation convenience and not related to spec at all.  When we
implemented this part the spec was saying we should return null in such a case
(http://www.w3.org/TR/2012/WD-FileAPI-20120712/#dfn-lastModifiedDate), but looks
like the latest spec says we should set Date.now() in the case?

[pwnall-personal, 2013/11/18 12:48:57]

That is my understanding of the spec too.

I think there is one source of confusion in our codebase, namely
invalidFileTime() / isValidFileTime() in Source/platform/FileMetadata.h.

The existence of this API makes me believe that the value used to represent an
invalid file time can be changed simply by changing the numbers here. In fact,
we have a few code paths that rely on this value being a NaN. At the same time,
it really makes sense to have NaN represent an invalid time, given how
JavaScript and WebIDL define invalid Dates. Should I add a comment?

[kinuko, 2013/11/18 13:27:09]

Sounds good.

+    //
+    // See step 3.3 in Section 7.1, and Section 7.2 of the File API spec:
+    // http://dev.w3.org/2006/webapi/FileAPI/
+    //
+    // See the following sections of the WebIDL ED:
+    // 3.10.26: http://heycam.github.io/webidl/#idl-Date
+    // 4.2.27: http://heycam.github.io/webidl/#es-Date
+
+    File* file = V8File::toNative(info.Holder());
+    v8SetReturnValue(info, v8::Date::New(file->lastModifiedDate()));
+}
+
 } // namespace WebCore