Allow package names to start with underscore.

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

Index: sdk/lib/_internal/pub/test/validator/name_test.dart
diff --git a/sdk/lib/_internal/pub/test/validator/name_test.dart b/sdk/lib/_internal/pub/test/validator/name_test.dart
index 6a70d11ff8b79433ba77040b1d7fedd8a1ce85c9..17a460fe02e3a5bef7f3ef78d26ee2e8749bfae7 100644
--- a/sdk/lib/_internal/pub/test/validator/name_test.dart
+++ b/sdk/lib/_internal/pub/test/validator/name_test.dart
@@ -33,6 +33,16 @@ main() {
       ]).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', () {