Use PUB_HOSTED_SERVER to set server URL for pub tests.

sdk/lib/_internal/pub/lib/src/source/hosted.dart

Index: sdk/lib/_internal/pub/lib/src/source/hosted.dart
diff --git a/sdk/lib/_internal/pub/lib/src/source/hosted.dart b/sdk/lib/_internal/pub/lib/src/source/hosted.dart
index f65a56a861f417b1b07d64f39a3fdf27c2a4bce6..aec150c888026320a7fa567d5c772641c84cbf86 100644
--- a/sdk/lib/_internal/pub/lib/src/source/hosted.dart
+++ b/sdk/lib/_internal/pub/lib/src/source/hosted.dart
@@ -31,7 +31,11 @@ class HostedSource extends Source {
   /// Gets the default URL for the package server for hosted dependencies.
   static String get defaultUrl {
     var url = io.Platform.environment["PUB_HOSTED_URL"];
-    if (url != null) return url;
+    if (url != null) {
+      log.fine("Got server $url from PUB_HOSTED_URL.");
+      return url;
+    }
+
     return "https://pub.dartlang.org";
   }