vim-patch:8.1.1354: getting a list of text lines is clumsy

Problem:    Getting a list of text lines is clumsy.
Solution:   Add the =<< assignment. (Yegappan Lakshmanan, closes vim/vim#4386)
f5842c5a53

src/nvim/testdir/test_let.vim

@@ -140,3 +140,58 @@ func Test_let_varg_fail()
   call assert_fails('call s:set_varg7(1)', 'E742:')
   call s:set_varg8([0])
 endfunction
+
+
+" Test for the setting a variable using the heredoc syntax
+func Test_let_heredoc()
+  let var1 =<< END
+Some sample text
+	Text with indent
+  !@#$%^&*()-+_={}|[]\~`:";'<>?,./
+END
+
+  call assert_equal(["Some sample text", "\tText with indent", "  !@#$%^&*()-+_={}|[]\\~`:\";'<>?,./"], var1)
+
+  let var2 =<<
+Editor
+.
+  call assert_equal(['Editor'], var2)
+
+  let var3 =<<END
+END
+  call assert_equal([], var3)
+
+  let var3 =<<END
+vim
+
+end
+  END
+END 
+END
+  call assert_equal(['vim', '', 'end', '  END', 'END '], var3)
+
+        let var1 =<< trim END
+        Line1
+          Line2
+        	Line3
+         END
+        END
+  call assert_equal(['Line1', '  Line2', "\tLine3", ' END'], var1)
+
+  let var1 =<< trim
+    Line1
+  .
+  call assert_equal(['  Line1'], var1)
+
+  call assert_fails('let v =<< marker', 'E991:')
+  call assert_fails('call WrongSyntax()', 'E488:')
+  call assert_fails('call MissingEnd()', 'E990:')
+endfunc
+
+func WrongSyntax()
+  let fail =<< that there
+endfunc
+
+func MissingEnd()
+  let fail =<< END
+endfunc