Documenting misleading DataTransfer API.

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index f1fff678094758c2f49c8855c5f1289e9433c2f2..4bf8f7555a90bd92b90a1170d2ade6a59b710203 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -5546,6 +5546,22 @@ class DataTransfer native "*Clipboard" {
   @DocsEditable
   void clearData([String type]) native;
 
+  /**
+   * Gets the data for the specified type.
+   *
+   * The data is only available from within a drop operation (such as an
+   * [Element.onDrop] event) and will return `null` before the event is
+   * triggered.
+   *
+   * Data transfer is prohibited across domains. If a drag originates
+   * from content from another domain or protocol (HTTP vs HTTPS) then the
+   * data cannot be accessed.
+   *
+   * The [type] can have values such as:
+   *
+   * * `'Text'`
+   * * `'URL'`
+   */
   @DomName('Clipboard.getData')
   @DocsEditable
   String getData(String type) native;