Update HTML5 File APIs

[lexaknyazev]

Existing JS bindings (as of 1.25.0-dev.16.4) for HTML 5 Directories and System File API are based on a discontinued spec. There's a fairly supported (at least in Chrome and Firefox) draft spec based on discontinued one: File and Directory Entries API.

Also, likely due to changed names (e.g., DirectoryEntry to FileSystemDirectoryEntry), DDC fails on runtime checks:

final items = e.dataTransfer.items;
for (var i = 0; i < items.length; ++i) {
  final entry = items[i].getAsEntry();
  if (entry.isDirectory) {
    final dir = entry as DirectoryEntry; // <-- DDC fails
    //...
  }
}

[alan-knight]

We have been lagging on the version of Chrome IDL we use, and it's currently at 50. It should be bumping up to 60 pretty soon. In version 50, DirectoryEntry is there. But it isn't clear to me that even in the latest version that we have FileSystemDirectoryEntry. It seems like it might be there under the old name, as a NoInterfaceObject? There's an intent to change that, but it doesn't seem to have happened. https://bugs.chromium.org/p/chromium/issues/detail?id=652814

[lexaknyazev]

Hmm. Those interfaces seem to work in Chrome as they're defined in the current spec but it's impossible to refer to them by names (nor old, neither new), so instanceof doesn't work. Dumping objects via console.log reveals that Chrome uses old names internally.