Pool future creation to ensure no more than 10 ops are in flight

pkg/barback/lib/src/phase.dart

Index: pkg/barback/lib/src/phase.dart
diff --git a/pkg/barback/lib/src/phase.dart b/pkg/barback/lib/src/phase.dart
index b83061bb96a1a200fd730319aef32d4c70375978..14d236a9b784264a795bd504e75c62680e335908 100644
--- a/pkg/barback/lib/src/phase.dart
+++ b/pkg/barback/lib/src/phase.dart
@@ -273,19 +273,15 @@ class Phase {
       }
     }
 
-    var outputFutures = [];
-    outputFutures.addAll(_inputs.values.map((input) {
-      if (!input.isDirty) return new Future.value(new Set());
-      return input.process().then(_handleOutputs);
-    }));
-    outputFutures.addAll(_groups.values.map((input) {
-      if (!input.isDirty) return new Future.value(new Set());
-      return input.process().then(_handleOutputs);
-    }));
+    var outputs = new List.from(_inputs.values);
+    outputs.addAll(_groups.values);
 
     // TODO(nweiz): handle pass-through.
 
-    return Future.wait(outputFutures).then((_) {
+    return forEachPooledFuture(outputs, (input) {
+      if (!input.isDirty) return null;
+      return input.process().then(_handleOutputs);
+    }).then((_) {
       // Report collisions in a deterministic order.
       outputIds = outputIds.toList();
       outputIds.sort((a, b) => a.compareTo(b));