Support transformers that depend on other transformers.

sdk/lib/_internal/pub/lib/src/package_graph.dart

Index: sdk/lib/_internal/pub/lib/src/package_graph.dart
diff --git a/sdk/lib/_internal/pub/lib/src/package_graph.dart b/sdk/lib/_internal/pub/lib/src/package_graph.dart
index 687b7202ace05cbed9dd68757723106d6a4213fb..634eb5ce5918cbda59e756ed339b564406472cb8 100644
--- a/sdk/lib/_internal/pub/lib/src/package_graph.dart
+++ b/sdk/lib/_internal/pub/lib/src/package_graph.dart
@@ -18,4 +18,22 @@ class PackageGraph {
   final Map<String, Package> packages;
 
   PackageGraph(this.entrypoint, this.packages);
+
+  /// Returns the set of transitive dependencies of the package named
+  /// [packageName].
+  Set<Package> transitiveDependencies(String packageName) {
+    var seen = new Set<Package>();
+    traverse(Package package) {
+      if (seen.contains(package)) return;
+      seen.add(package);
+      for (var dep in package.dependencies) {
+        traverse(packages[dep.name]);
+      }
+    }
+
+    var package = packages[packageName];
+    traverse(package);
+    seen.remove(package);
+    return seen;
+  }
 }