Deal better with missing sources in code_transformers and polymer

pkg/code_transformers/test/resolver_test.dart

Index: pkg/code_transformers/test/resolver_test.dart
diff --git a/pkg/code_transformers/test/resolver_test.dart b/pkg/code_transformers/test/resolver_test.dart
index 1403b1f666246431449941480a74f9174b68e90a..bdf8a68e709ea9bd5280cf7a62def0dd97fef7cf 100644
--- a/pkg/code_transformers/test/resolver_test.dart
+++ b/pkg/code_transformers/test/resolver_test.dart
@@ -127,6 +127,9 @@ main() {
               main() {
               } ''',
           },
+          messages: [
+            'warning: Asset not found: b|lib/missing.dart',
+          ],
           validator: (resolver) {
             var lib = resolver.getLibrary(entryPoint);
             expect(lib.importedLibraries.length, 1);