Add native extension to flutter targets (this was removed accidentally in an

pkg/kernel/lib/target/flutter.dart

Index: pkg/kernel/lib/target/flutter.dart
diff --git a/pkg/kernel/lib/target/flutter.dart b/pkg/kernel/lib/target/flutter.dart
index 79b307156554150d54d84a4e259f71f8421dcc1a..ea24427f61c47b6678dcba697351050e0864be3c 100644
--- a/pkg/kernel/lib/target/flutter.dart
+++ b/pkg/kernel/lib/target/flutter.dart
@@ -87,4 +87,12 @@ class FlutterTarget extends Target {
     // TODO(ahe): This should probably return the same as VmTarget does.
     return new InvalidExpression();
   }
+
+  // TODO(sigmund,ahe): limit this to `dart-ext` libraries only (see
+  // https://github.com/dart-lang/sdk/issues/29763).
+  @override
+  bool enableNative(Uri uri) => true;
+
+  @override
+  bool get nativeExtensionExpectsString => true;
 }