[Files app] ES6 class for file_system_metadata_provider.js

Bug: 778674
Change-Id: I26adb7ae525b1965280b60479095de3e8a32562e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1647640
Commit-Queue: François Degros <fdegros@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Auto-Submit: François Degros <fdegros@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666613}

ui/file_manager/file_manager/foreground/js/metadata/file_system_metadata_provider.js

@@ -4,27 +4,15 @@
 
 /**
  * Metadata provider for FileEntry#getMetadata.
- *
- * @constructor
- * @extends {MetadataProvider}
- * @struct
+ * @final
  */
-function FileSystemMetadataProvider() {
-  MetadataProvider.call(this, FileSystemMetadataProvider.PROPERTY_NAMES);
-}
-
-/**
- * @const {!Array<string>}
- */
-FileSystemMetadataProvider.PROPERTY_NAMES =
-    ['modificationTime', 'size', 'present', 'availableOffline'];
-
-FileSystemMetadataProvider.prototype.__proto__ = MetadataProvider.prototype;
+class FileSystemMetadataProvider extends MetadataProvider {
+  constructor() {
+    super(FileSystemMetadataProvider.PROPERTY_NAMES);
+  }
 
-/**
- * @override
- */
-FileSystemMetadataProvider.prototype.get = requests => {
+  /** @override */
+  get(requests) {
     if (!requests.length) {
       return Promise.resolve([]);
     }
@@ -43,11 +31,16 @@ FileSystemMetadataProvider.prototype.get = requests => {
               },
               error => {
                 // Can't use console.error because some tests hit this line and
-              // console.error causes them to fail because of JSErrorCount. This
-              // error is an acceptable condition.
+                // console.error causes them to fail because of JSErrorCount.
+                // This error is an acceptable condition.
                 console.warn(
                     'getMetadata failure for: ' + request.entry.toURL(), error);
                 return new MetadataItem();
               });
     }));
-};
+  }
+}
+
+/** @const {!Array<string>} */
+FileSystemMetadataProvider.PROPERTY_NAMES =
+    ['modificationTime', 'size', 'present', 'availableOffline'];