Use helper methods for running pub.

sdk/lib/_internal/pub/test/install/switch_source_test.dart

Index: sdk/lib/_internal/pub/test/install/switch_source_test.dart
diff --git a/sdk/lib/_internal/pub/test/install/switch_source_test.dart b/sdk/lib/_internal/pub/test/install/switch_source_test.dart
index db8864e54e16ec6837e32791a171c30cfdaa446a..428292e82849291f87496ddfaeba5effe88fec0d 100644
--- a/sdk/lib/_internal/pub/test/install/switch_source_test.dart
+++ b/sdk/lib/_internal/pub/test/install/switch_source_test.dart
@@ -19,17 +19,13 @@ main() {
 
     d.appDir([{"path": "../foo"}]).create();
 
-    schedulePub(args: ['install'],
-        output: new RegExp("Dependencies installed!\$"));
+    pubInstall();
 
     d.packagesDir({"foo": "0.0.1"}).validate();
-
     servePackages([packageMap("foo", "1.2.3")]);
-
     d.appDir([dependencyMap("foo", "any")]).create();
 
-    schedulePub(args: ['install'],
-        output: new RegExp("Dependencies installed!\$"));
+    pubInstall();
 
     d.packagesDir({"foo": "1.2.3"}).validate();
   });