patch 9.0.0356: :echowindow sets the in_echowindow flag too early

Problem:    :echowindow sets the in_echowindow flag too early.
Solution:   Set in_echowindow only when outputting the text. (Yasuhiro
            Matsumoto, closes #11033)
diff --git a/src/testdir/dumps/Test_echowin_eval.dump b/src/testdir/dumps/Test_echowin_eval.dump
new file mode 100644
index 0000000..80d5fc7
--- /dev/null
+++ b/src/testdir/dumps/Test_echowin_eval.dump
@@ -0,0 +1,8 @@
+> +0&#ffffff0@74
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|~| @73
+|═+0#e000002&@74
+|t|e|s|t| @70
+|1+0#0000000&|2|3| @53|0|,|0|-|1| @8|A|l@1|