patch 8.2.0336: Vim9: insufficient test coverage for compiling

Problem:    Vim9: insufficient test coverage for compiling.
Solution:   Add more tests.
diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index 5ece652..1ec3c6f 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -9,6 +9,12 @@
   call delete('Xdef')
 endfunc
 
+func CheckDefFailureMult(lines, error)
+  call writefile(['def! Func()'] + a:lines + ['enddef'], 'Xdef')
+  call assert_fails('so Xdef', a:error, join(a:lines, ' | '))
+  call delete('Xdef')
+endfunc
+
 " Check that "line" inside ":def" results in an "error" message when executed.
 func CheckDefExecFailure(line, error)
   call writefile(['def! Func()', a:line, 'enddef'], 'Xdef')
@@ -805,6 +811,8 @@
   call CheckDefExecFailure("let x = +g:ablob", 'E974:')
   call CheckDefExecFailure("let x = +g:alist", 'E745:')
   call CheckDefExecFailure("let x = +g:adict", 'E728:')
+
+  call CheckDefFailureMult(["let x = ''", "let y = x.memb"], 'E715:')
 endfunc
 
 let g:Funcrefs = [function('add')]