Move enableNative to target, turn it on by default

pkg/front_end/test/subpackage_relationships_test.dart

Index: pkg/front_end/test/subpackage_relationships_test.dart
diff --git a/pkg/front_end/test/subpackage_relationships_test.dart b/pkg/front_end/test/subpackage_relationships_test.dart
index 0f96056ad4e15088bf24f9bef5a811e425e623f4..354a5e99597d87dcea322bb41cf47912920d1376 100644
--- a/pkg/front_end/test/subpackage_relationships_test.dart
+++ b/pkg/front_end/test/subpackage_relationships_test.dart
@@ -54,6 +54,7 @@ final subpackageRules = {
     'lib/src/fasta/kernel',
     'lib/src/fasta/parser',
     'lib/src/fasta/scanner',
+    'lib/src/fasta/source',

[ahe, 2017/05/31 09:07:50]

OK. Since this is the first use of source in this directory, I'm going to say
it's a really bad idea to use SourceLibraryBuilder.

     'lib/src/fasta/testing',
     'lib/src/fasta/util',
     'lib/src/scanner',