fixes #259, clone ast before mutation

test/codegen/expect/syncstar_yield_test.txt

Index: test/codegen/expect/syncstar_yield_test.txt
diff --git a/test/codegen/expect/syncstar_yield_test.txt b/test/codegen/expect/syncstar_yield_test.txt
index e41a0e3c0d157b31fe865e845a61625938dd55b8..f8f11136ec7c7338e45a7825a190e4171dd4dcac 100644
--- a/test/codegen/expect/syncstar_yield_test.txt
+++ b/test/codegen/expect/syncstar_yield_test.txt
@@ -1,3 +1,5 @@
 // Messages from compiling syncstar_yield_test.dart
+info: [DynamicCast] expected (dynamic) will need runtime check to cast to type String (test/codegen/expect.dart, line 97, col 51)

[Jennifer Messerly, 2015/07/21 19:58:46]

these messages were skipped before because we didn't recompile "expect.dart"
when we started the compilation for "syncstar_yield_test.dart". This is an
artifact of codegen_test running multiple BatchCompiler instances against the
same AnalysisContext (which it should stop doing--though for the moment it's
improving our test coverage of analysis server use case)

+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] p (dynamic) will need runtime check to cast to type num (test/codegen/syncstar_yield_test.dart, line 17, col 28)