Add regression test for #1645. (#1697)

This is already fixed (probably by #1687).

Close #1645.

Author: munificent

test/tall/regression/1600/1645.unit

@@ -0,0 +1,13 @@
+>>>
+Future<String> fetchUserOrder() => Future.delayed(const Duration(seconds: 2), () => 'Large Latte');
+<<<
+Future<String> fetchUserOrder() =>
+    Future.delayed(const Duration(seconds: 2), () => 'Large Latte');
+>>>
+Future<String> fetchUserOrder() =>
+    // Imagine that this function is more complex and slow.
+    Future.delayed(const Duration(seconds: 2), () => 'Large Latte');
+<<<
+Future<String> fetchUserOrder() =>
+    // Imagine that this function is more complex and slow.
+    Future.delayed(const Duration(seconds: 2), () => 'Large Latte');