Move enableNative to target, turn it on by default

pkg/front_end/test/subpackage_relationships_test.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:front_end/compiler_options.dart';
 import 'package:front_end/dependency_grapher.dart';
 import 'package:path/path.dart' as pathos;
@@ skipping 36 matching lines @@
     'lib/src/incremental'
   ]),
   'lib/src/codegen': new SubpackageRules(),
   'lib/src/fasta': new SubpackageRules(allowedDependencies: [
     'lib',
     'lib/src/fasta/builder',
     'lib/src/fasta/dill',
     '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',
   ]),
   'lib/src/fasta/builder': new SubpackageRules(allowedDependencies: [
     'lib/src/fasta',
     'lib/src/fasta/parser',
     'lib/src/fasta/source',
     'lib/src/fasta/util',
   ]),
@@ skipping 226 matching lines @@
       if (subpackageRules[subpackage].allowSubdirs) {
         subpackageRules[subpackage].actuallyHasSubdirs = true;
       } else {
         problem('Uri $src is in a subfolder of $subpackage, but that '
             'subpackage does not allow dart files in subdirectories.');
       }
     }
     return subpackage;
   }
 }