Move enableNative to target, turn it on by default

pkg/front_end/lib/src/fasta/target_implementation.dart

 // Copyright (c) 2016, 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.
 
 library fasta.target_implementation;
 
 import 'package:kernel/target/targets.dart' as backend show Target;
 
 import 'builder/builder.dart' show Builder, ClassBuilder, LibraryBuilder;
 
@@ skipping 70 matching lines @@
     LibraryBuilder internal = loader.read(Uri.parse("dart:_internal"), -1);
     return cachedNativeAnnotation = internal.getConstructor("ExternalName");
   }
 
   void loadExtraRequiredLibraries(Loader loader) {
     for (String uri in backendTarget.extraRequiredLibraries) {
       loader.read(Uri.parse(uri), -1);
     }
   }
 
+  /// Whether the `native` language extension is supported within [library].
+  ///
+  /// The `native` language extension is not part of the language specification,
+  /// means something else to each target, and is enabled differently for each
+  /// target implementation. For example, the VM target enables it everywhere
+  /// because of existing support for "dart-ext:" native extensions, but targets
+  /// like dart2js only enable it on the core libraries.
+  ///
+  /// This default implementation assumes a VM target, but it can be overriden
+  /// in subclasses to change the behavior.
+  // TODO(sigmund,ahe): limit this to `dart-ext` libraries only (see
+  // https://github.com/dart-lang/sdk/issues/29763).
+  bool enableNative(LibraryBuilder library) => true;
+
   Token skipNativeClause(Token token) => vm.skipNativeClause(token);
 
   String extractNativeMethodName(Token token) =>
       unescapeString(token.next.lexeme);
 
   void addSourceInformation(
       Uri uri, List<int> lineStarts, List<int> sourceCode);
 
   void readPatchFiles(LibraryBuilder library) {
     assert(library.uri.scheme == "dart");
     List<Uri> patches = uriTranslator.patches[library.uri.path];
     if (patches != null) {
       for (Uri patch in patches) {
         library.loader.read(patch, -1, fileUri: patch, isPatch: true);
       }
     }
   }
 }