Create binstubs for executables when activating a package.

sdk/lib/_internal/pub_generated/test/validator/executable_test.dart

+import 'package:scheduled_test/scheduled_test.dart';
+import '../../lib/src/entrypoint.dart';
+import '../../lib/src/validator.dart';
+import '../../lib/src/validator/executable.dart';
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+import 'utils.dart';
+Validator executable(Entrypoint entrypoint) =>
+    new ExecutableValidator(entrypoint);
+main() {
+  initConfig();
+  setUp(d.validPackage.create);
+  group('should consider a package valid if it', () {
+    integration('has executables that are present', () {
+      d.dir(appPath, [d.pubspec({
+          "name": "test_pkg",
+          "version": "1.0.0",
+          "executables": {
+            "one": "one_script",
+            "two": null
+          }
+        }),
+            d.dir(
+                "bin",
+                [
+                    d.file("one_script.dart", "main() => print('ok');"),
+                    d.file("two.dart", "main() => print('ok');")])]).create();
+      expectNoValidationError(executable);
+    });
+  });
+  group("should consider a package invalid if it", () {
+    integration('is missing one or more listed executables', () {
+      d.dir(appPath, [d.pubspec({
+          "name": "test_pkg",
+          "version": "1.0.0",
+          "executables": {
+            "nope": "not_there",
+            "nada": null
+          }
+        })]).create();
+      expectValidationWarning(executable);
+    });
+  });
+}