Allow ".dart" extension in pub run.

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

Index: sdk/lib/_internal/pub/test/run/allows_dart_extension_test.dart
diff --git a/sdk/lib/_internal/pub/test/run/runs_app_in_entrypoint_test.dart b/sdk/lib/_internal/pub/test/run/allows_dart_extension_test.dart
similarity index 86%
copy from sdk/lib/_internal/pub/test/run/runs_app_in_entrypoint_test.dart
copy to sdk/lib/_internal/pub/test/run/allows_dart_extension_test.dart
index e3cb233f8634b569adf10d2b57c82f082ae93fa2..38bdb7716d3cdd4a7efea7e266ed5884ce730c44 100644
--- a/sdk/lib/_internal/pub/test/run/runs_app_in_entrypoint_test.dart
+++ b/sdk/lib/_internal/pub/test/run/allows_dart_extension_test.dart
@@ -17,7 +17,7 @@ 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", [
@@ -25,7 +25,7 @@ main() {
       ])
     ]).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);