Include directories in pub build usage.

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 4ab5f22a6986b7326f83c8aaabdf4ab0d42bf8da..b4318c36540e4354d4bb64676d8ce8541ea5c2a9 100644
--- a/sdk/lib/_internal/pub/lib/src/command/build.dart
+++ b/sdk/lib/_internal/pub/lib/src/command/build.dart
@@ -27,7 +27,7 @@ final _allowedBuildDirectories = new Set<String>.from([
 /// Handles the `build` pub command.
 class BuildCommand extends PubCommand {
   String get description => "Apply transformers to build a package.";
-  String get usage => "pub build [options]";
+  String get usage => "pub build [options] [directories...]";
   List<String> get aliases => const ["deploy", "settle-up"];
   bool get takesArguments => true;