Use helper methods for running pub.

sdk/lib/_internal/pub/test/install/hosted/stay_locked_test.dart

Index: sdk/lib/_internal/pub/test/install/hosted/stay_locked_test.dart
diff --git a/sdk/lib/_internal/pub/test/install/hosted/stay_locked_test.dart b/sdk/lib/_internal/pub/test/install/hosted/stay_locked_test.dart
index 188012e719ab85f3bc643aacdf92ac402dc33b3a..4cbafe390253c6cbd3d5405f06f415be56acd311 100644
--- a/sdk/lib/_internal/pub/test/install/hosted/stay_locked_test.dart
+++ b/sdk/lib/_internal/pub/test/install/hosted/stay_locked_test.dart
@@ -22,8 +22,7 @@ main() {
     d.appDir([dependencyMap("foo")]).create();
 
     // This install should lock the foo dependency to version 1.0.0.
-    schedulePub(args: ['install'],
-        output: new RegExp(r"Dependencies installed!$"));
+    pubInstall();
 
     d.packagesDir({"foo": "1.0.0"}).validate();
 
@@ -34,8 +33,7 @@ main() {
     servePackages([packageMap("foo", "1.0.1")]);
 
     // This install shouldn't update the foo dependency due to the lockfile.
-    schedulePub(args: ['install'],
-        output: new RegExp(r"Dependencies installed!$"));
+    pubInstall();
 
     d.packagesDir({"foo": "1.0.0"}).validate();
   });