Add an extension point for contributing to HTML_ERRORS.

pkg/analyzer/lib/plugin/task.dart

Index: pkg/analyzer/lib/plugin/task.dart
diff --git a/pkg/analyzer/lib/plugin/task.dart b/pkg/analyzer/lib/plugin/task.dart
index 45141a2e8836ef267fed0a01f1450993e0cb3d35..fd4dd7cd0d8a7c79af9bbd3be4614d076942a6e5 100644
--- a/pkg/analyzer/lib/plugin/task.dart
+++ b/pkg/analyzer/lib/plugin/task.dart
@@ -33,6 +33,14 @@ final String DART_ERRORS_FOR_UNIT_EXTENSION_POINT_ID = Plugin.join(
 
 /**
  * The identifier of the extension point that allows plugins to register new
+ * analysis error results to compute for an HTML source. The object used as an
+ * extension must be a [ResultDescriptor].
+ */
+final String HTML_ERRORS_EXTENSION_POINT_ID = Plugin.join(
+    EnginePlugin.UNIQUE_IDENTIFIER, EnginePlugin.HTML_ERRORS_EXTENSION_POINT);
+
+/**
+ * The identifier of the extension point that allows plugins to register new
  * analysis tasks with the analysis engine. The object used as an extension must
  * be a [TaskDescriptor].
  */