Make the Flow type of CompositeDraftDecorator's constructor more strict

Summary: The input array of decorators gets cloned in the constructor to prevent mutations of the original array from affecting the composite. Another way to communicate that the input array should not be mutated is with the `$ReadOnlyArray<...>` Flow type.

Reviewed By: kassens

Differential Revision: D6882262

fbshipit-source-id: 35bfb957a7d006d5554d6ba11ba7d095800d40b0

Author: steveluscher

src/model/decorators/CompositeDraftDecorator.js

@@ -43,9 +43,9 @@ var DELIMITER = '.';
  * preserved and the new match is discarded.
  */
 class CompositeDraftDecorator {
-  _decorators: Array<DraftDecorator>;
+  _decorators: $ReadOnlyArray<DraftDecorator>;
 
-  constructor(decorators: Array<DraftDecorator>) {
+  constructor(decorators: $ReadOnlyArray<DraftDecorator>) {
     // Copy the decorator array, since we use this array order to determine
     // precedence of decoration matching. If the array is mutated externally,
     // we don't want to be affected here.