Create a public API for contributing assists and make assists pluggable

pkg/analysis_server/lib/src/services/correction/fix_internal.dart

Index: pkg/analysis_server/lib/src/services/correction/fix_internal.dart
diff --git a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
index 4f4b936c1a08516437898756f9ee4087c935dbae..eb9c8357dd6c02c054fcb77e710e99eff051a1e7 100644
--- a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
+++ b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
@@ -40,8 +40,7 @@ import 'package:path/path.dart';
 typedef bool ElementPredicate(Element argument);
 
 /**
- * A sketch of an implementation of the current FixProcessor (in fix_internal)
- * as a fix contributor.
+ * A [FixContributor] that provides the default set of fixes.
  */
 class DefaultFixContributor extends DartFixContributor {
   @override