Remove a fixed size list creation (#138)

The dart idiom for a repeated string is `operator *`. Remove an
unnecessary intermediate list and `String.fromCharCodes`.
diff --git a/test/support.dart b/test/support.dart
index 34fedb2..ee2557a 100644
--- a/test/support.dart
+++ b/test/support.dart
@@ -110,12 +110,7 @@
 
   set indent(int value) {
     if (_indent == value) return;
-
-    final arr = List<int>(value);
-    for (var i = 0; i < value; i++) {
-      arr[i] = 32;
-    }
-    _spaces = String.fromCharCodes(arr);
+    _spaces = ' ' * value;
     _indent = value;
   }