js: Add IntDict class.

We don't use this yet, but we will soon.

We report errors if users pass in strings instead of
ints, but we try to still use the key.

tools/test-js-with-node

@@ -51,6 +51,7 @@ enforce_fully_covered = {
     'static/js/hash_util.js',
     'static/js/keydown_util.js',
     'static/js/input_pill.js',
+    'static/js/int_dict.ts',
     'static/js/list_cursor.js',
     'static/js/markdown.js',
     'static/js/message_store.js',