Support serving from multiple directories using "pub serve".

sdk/lib/_internal/pub/lib/src/command/build.dart

Index: sdk/lib/_internal/pub/lib/src/command/build.dart
diff --git a/sdk/lib/_internal/pub/lib/src/command/build.dart b/sdk/lib/_internal/pub/lib/src/command/build.dart
index eba1a101e0f0972a4a3791904ec47eed019254a4..aee823a5317db35af6ead95d1b1d0bf4d811b0af 100644
--- a/sdk/lib/_internal/pub/lib/src/command/build.dart
+++ b/sdk/lib/_internal/pub/lib/src/command/build.dart
@@ -68,12 +68,12 @@ class BuildCommand extends PubCommand {
 
       // Show in-progress errors, but not results. Those get handled implicitly
       // by getAllAssets().
-      environment.server.barback.errors.listen((error) {
+      environment.barback.errors.listen((error) {
         log.error(log.red("Build error:\n$error"));
       });
 
       return log.progress("Building ${entrypoint.root.name}",
-          () => environment.server.barback.getAllAssets()).then((assets) {
+          () => environment.barback.getAllAssets()).then((assets) {
         // Find all of the JS entrypoints we built.
         var dart2JSEntrypoints = assets
             .where((asset) => asset.id.path.endsWith(".dart.js"))