Allow ".dart" extension in pub run.

sdk/lib/_internal/pub/lib/src/executable.dart

Index: sdk/lib/_internal/pub/lib/src/executable.dart
diff --git a/sdk/lib/_internal/pub/lib/src/executable.dart b/sdk/lib/_internal/pub/lib/src/executable.dart
index 34482e36b7a77a432703531721df2af88df9d807..2becb86920e12dc18f4914507da7c87b77d5cbef 100644
--- a/sdk/lib/_internal/pub/lib/src/executable.dart
+++ b/sdk/lib/_internal/pub/lib/src/executable.dart
@@ -21,9 +21,9 @@ import 'utils.dart';
 /// Runs [executable] from [package] reachable from [entrypoint].
 ///
 /// The executable string is a relative Dart file path using native path
-/// separators without a trailing ".dart" extension. It is contained within
-/// [package], which should either be the entrypoint package or an immediate
-/// dependency of it.
+/// separators with or without a trailing ".dart" extension. It is contained
+/// within [package], which should either be the entrypoint package or an
+/// immediate dependency of it.
 ///
 /// Arguments from [args] will be passed to the spawned Dart application.
 ///
@@ -42,6 +42,11 @@ Future<int> runExecutable(Entrypoint entrypoint, String package,
     log.verbosity = log.Verbosity.WARNING;
   }
 
+  // Ignore a trailing extension.
+  if (p.extension(executable) == ".dart") {
+    executable = p.withoutExtension(executable);
+  }
+
   var localSnapshotPath = p.join(".pub", "bin", package,
       "$executable.dart.snapshot");
   if (!isGlobal && fileExists(localSnapshotPath) &&