Propagate canUseNative flag to synthesized library.

lib/compiler/implementation/unparse_validator.dart

Index: lib/compiler/implementation/unparse_validator.dart
diff --git a/lib/compiler/implementation/unparse_validator.dart b/lib/compiler/implementation/unparse_validator.dart
index 5004894e6e1f7a3bb043446e63fd1e790d944cdb..146811bbf849be66fefc843a1842230b370fa7c6 100644
--- a/lib/compiler/implementation/unparse_validator.dart
+++ b/lib/compiler/implementation/unparse_validator.dart
@@ -59,6 +59,7 @@ class UnparseValidator extends CompilerTask {
     Script synthesizedScript =
         new Script(originalScript.uri, synthesizedSourceFile);
     LibraryElement lib = new LibraryElement(synthesizedScript);
+    lib.canUseNative = element.getLibrary().canUseNative;
     NodeListener listener =
         new NodeListener(new ValidatorListener(synthesizedSourceFile), lib);
     parser = new Parser(listener);