patch 8.2.1322: Vim9: method on double quoted string doesn't work

Problem:    Vim9: method on double quoted string doesn't work.
Solution:   Recognize double quoted string. (closes #6562)
diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index 5b46c32..e29ef5c 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -1638,8 +1638,9 @@
 def Test_expr7_method_call()
   new
   setline(1, ['first', 'last'])
-  eval 'second'->append(1)
-  assert_equal(['first', 'second', 'last'], getline(1, '$'))
+  'second'->append(1)
+  "third"->append(2)
+  assert_equal(['first', 'second', 'third', 'last'], getline(1, '$'))
   bwipe!
 
   let bufnr = bufnr()