Move enableNative to target, turn it on by default

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

Index: pkg/front_end/lib/src/fasta/target_implementation.dart
diff --git a/pkg/front_end/lib/src/fasta/target_implementation.dart b/pkg/front_end/lib/src/fasta/target_implementation.dart
index 708795af65bb2c1722f76b4e9d4e9c4b58a04487..6d3ef741747c5e420f9a1ebab28b1ec377616b19 100644
--- a/pkg/front_end/lib/src/fasta/target_implementation.dart
+++ b/pkg/front_end/lib/src/fasta/target_implementation.dart
@@ -88,6 +88,20 @@ abstract class TargetImplementation extends Target {
     }
   }
 
+  /// 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) =>