First stab at "pub run" command.

sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart

Index: sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart b/sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart
index cea9ca279d391c3b3fc0f901608c3bcc4baec72c..c80bceac2fc330e4095b4b3b5f3d7d5588d073f5 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart
@@ -38,17 +38,15 @@ class AssetEnvironment {
   /// Creates a new build environment for working with the assets used by
   /// [entrypoint] and its dependencies.
   ///
-  /// Spawns an HTTP server for each directory in [rootDirectories]. These
-  /// servers will be on [hostname] and have ports based on [basePort].
-  /// [basePort] itself is reserved for "web/" and `basePort + 1` is reserved
-  /// for "test/"; further ports will be allocated for other root directories as
-  /// necessary. If [basePort] is zero, each server will have an ephemeral port.
+  /// HTTP servers that serve directories from this environment will be bound
+  /// to [hostname] and have ports based on [basePort]. If omitted, they
+  /// default to "127.0.0.1" and "0" (use ephemeral ports), respectively.
   ///
   /// Loads all used transformers using [mode] (including dart2js if
   /// [useDart2JS] is true).
   ///
-  /// Includes [rootDirectories] in the root package, as well as "lib" and
-  /// "asset".
+  /// This will only add the root package's "lib" directory to the environment.
+  /// Other directories can be added to the environment using [serveDirectory].
   ///
   /// If [watcherType] is not [WatcherType.NONE], watches source assets for
   /// modification.
@@ -56,8 +54,11 @@ class AssetEnvironment {
   /// Returns a [Future] that completes to the environment once the inputs,
   /// transformers, and server are loaded and ready.
   static Future<AssetEnvironment> create(Entrypoint entrypoint,
-      String hostname, int basePort, BarbackMode mode, WatcherType watcherType,
-      {bool useDart2JS: true}) {
+      BarbackMode mode, WatcherType watcherType,
+      {String hostname, int basePort, bool useDart2JS: true}) {
+    if (hostname == null) hostname = "127.0.0.1";
+    if (basePort == null) basePort = 0;
+
     return entrypoint.loadPackageGraph().then((graph) {
       log.fine("Loaded package graph.");
       var barback = new Barback(new PubPackageProvider(graph));