Documenting misleading DataTransfer API.

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index 88901637363ec9dae67fe6c3750356ccf8ae596a..95e11ec44aba0e164498dac775f2918d9ebc993d 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -6496,6 +6496,21 @@ class DataTransfer extends NativeFieldWrapperClass1 {
   @DocsEditable
   void clearData([String type]) native "Clipboard_clearData_Callback";
 
+  /**
+   * Gets the data for the specified type.
+   *
+   * The  data is only available from within a drop operation (such as an
+   * onDrop event) and will return `null` before then.
+   *
+   * Data transfer is also 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 "Clipboard_getData_Callback";