fixes #259, clone ast before mutation

test/codegen/expect/syncstar_yieldstar_test.txt

Index: test/codegen/expect/syncstar_yieldstar_test.txt
diff --git a/test/codegen/expect/syncstar_yieldstar_test.txt b/test/codegen/expect/syncstar_yieldstar_test.txt
index c0d2c3646b596bc7cc505a3c484bfca298f1d488..4d14fecc5b1b30cdacfcb24d1aad2f3e435ba031 100644
--- a/test/codegen/expect/syncstar_yieldstar_test.txt
+++ b/test/codegen/expect/syncstar_yieldstar_test.txt
@@ -1,4 +1,6 @@
 // Messages from compiling syncstar_yieldstar_test.dart
+info: [DynamicCast] expected (dynamic) will need runtime check to cast to type String (test/codegen/expect.dart, line 97, col 51)
+info: [DynamicCast] actual (dynamic) will need runtime check to cast to type String (test/codegen/expect.dart, line 97, col 61)
 info: [DynamicInvoke] check(e) requires dynamic invoke (test/codegen/expect.dart, line 372, col 14)
 info: [DynamicCast] foo().take(9).toList() (dynamic) will need runtime check to cast to type List<dynamic> (test/codegen/syncstar_yieldstar_test.dart, line 28, col 53)
 info: [DynamicInvoke] foo().take(9).toList() requires dynamic invoke (test/codegen/syncstar_yieldstar_test.dart, line 28, col 53)