Allow ".dart" extension in pub run.

sdk/lib/_internal/pub_generated/test/run/allows_dart_extension_test.dart

Index: sdk/lib/_internal/pub_generated/test/run/allows_dart_extension_test.dart
diff --git a/sdk/lib/_internal/pub_generated/test/run/runs_app_in_entrypoint_test.dart b/sdk/lib/_internal/pub_generated/test/run/allows_dart_extension_test.dart
similarity index 80%
copy from sdk/lib/_internal/pub_generated/test/run/runs_app_in_entrypoint_test.dart
copy to sdk/lib/_internal/pub_generated/test/run/allows_dart_extension_test.dart
index 6ec3e8c76c45d395482ff30c0fdcf7b1595fed3e..2eaee2321359b6180940fd811ffded929998bfe9 100644
--- a/sdk/lib/_internal/pub_generated/test/run/runs_app_in_entrypoint_test.dart
+++ b/sdk/lib/_internal/pub_generated/test/run/allows_dart_extension_test.dart
@@ -11,11 +11,11 @@ main() {
 """;
 main() {
   initConfig();
-  integration('runs a Dart application in the entrypoint package', () {
+  integration('allows a ".dart" extension on the argument', () {
     d.dir(
         appPath,
         [d.appPubspec(), d.dir("bin", [d.file("script.dart", SCRIPT)])]).create();
-    var pub = pubRun(args: ["script"]);
+    var pub = pubRun(args: ["script.dart"]);
     pub.stdout.expect("stdout output");
     pub.stderr.expect("stderr output");
     pub.shouldExit(123);