Have barback lazily load assets from static pub packages.

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

Index: sdk/lib/_internal/pub/lib/src/barback/pub_package_provider.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/pub_package_provider.dart b/sdk/lib/_internal/pub/lib/src/barback/pub_package_provider.dart
index 72f22928d982d139d91170cc24212b27280ed607..a7c7046b05bb79d0f2faa9b0d59430cb25f21dfd 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/pub_package_provider.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/pub_package_provider.dart
@@ -17,13 +17,17 @@ import '../utils.dart';
 
 /// An implementation of barback's [PackageProvider] interface so that barback
 /// can find assets within pub packages.
-class PubPackageProvider implements PackageProvider {
+class PubPackageProvider implements StaticPackageProvider {
   final PackageGraph _graph;
-  final List<String> packages;
+  final List<String> staticPackages;
+
+  Iterable<String> get packages =>
+      _graph.packages.keys.toSet().difference(staticPackages);
 
   PubPackageProvider(PackageGraph graph)
       : _graph = graph,
-        packages = [r"$pub", r"$sdk"]..addAll(graph.packages.keys);
+        staticPackages = [r"$pub", r"$sdk"]..addAll(
+            graph.packages.keys.where(graph.isPackageStatic));
 
   Future<Asset> getAsset(AssetId id) {
     // "$pub" is a psuedo-package that allows pub's transformer-loading
@@ -69,4 +73,13 @@ class PubPackageProvider implements PackageProvider {
     var file = _graph.packages[id.package].path(nativePath);
     return new Future.value(new Asset.fromPath(id, file));
   }
+
+  Stream<AssetId> getAllAssetIds(String packageName) {
+    var package = _graph.packages[packageName];
+    return new Stream.fromIterable(
+        package.listFiles(beneath: 'lib').map((file) {
+      return new AssetId(packageName,
+          path.toUri(package.relative(file)).toString());
+    }));
+  }
 }