Allow package names to start with underscore.

sdk/lib/_internal/pub/test/validator/name_test.dart

 // Copyright (c) 2013, 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 'package:path/path.dart' as path;
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../../lib/src/entrypoint.dart';
 import '../../lib/src/io.dart';
 import '../../lib/src/validator.dart';
@@ skipping 15 matching lines @@
     integration('has a badly-named library in lib/src', () {
       d.dir(appPath, [
         d.libPubspec("test_pkg", "1.0.0"),
         d.dir("lib", [
           d.file("test_pkg.dart", "int i = 1;"),
           d.dir("src", [d.file("8ball.dart", "int j = 2;")])
         ])
       ]).create();
       expectNoValidationError(name);
     });
+
+    integration('has a name that starts with an underscore', () {
+      d.dir(appPath, [
+        d.libPubspec("_test_pkg", "1.0.0"),
+        d.dir("lib", [
+          d.file("_test_pkg.dart", "int i = 1;")
+        ])
+      ]).create();
+      expectNoValidationError(name);
+    });
   });
 
   group('should consider a package invalid if it', () {
     setUp(d.validPackage.create);
 
     integration('has an empty package name', () {
       d.dir(appPath, [d.libPubspec("", "1.0.0")]).create();
       expectValidationError(name);
     });
 
@@ skipping 52 matching lines @@
     integration('has a single library named differently than the package', () {
       schedule(() =>
           deleteEntry(path.join(sandboxDir, appPath, "lib", "test_pkg.dart")));
       d.dir(appPath, [
         d.dir("lib", [d.file("best_pkg.dart", "int i = 0;")])
       ]).create();
       expectValidationWarning(name);
     });
   });
 }