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

pkg/polymer/lib/src/build/runner.dart

Index: pkg/polymer/lib/src/build/runner.dart
diff --git a/pkg/polymer/lib/src/build/runner.dart b/pkg/polymer/lib/src/build/runner.dart
index 064ac089e4b14934ab5e234949bc0a02ab9a4da3..cada2d50db1973e4e22d2aa9d1d3396455de8d4c 100644
--- a/pkg/polymer/lib/src/build/runner.dart
+++ b/pkg/polymer/lib/src/build/runner.dart
@@ -14,6 +14,7 @@ import 'dart:io';
 
 import 'package:args/args.dart';
 import 'package:barback/barback.dart';
+import 'package:barback/src/utils.dart' show forEachPooledFuture;
 import 'package:path/path.dart' as path;
 import 'package:stack_trace/stack_trace.dart';
 import 'package:yaml/yaml.dart';
@@ -211,7 +212,7 @@ Future _emitTransformedFiles(AssetSet assets, BarbackOptions options) {
   var transformTests = options.transformTests;
   var outPackages = path.join(options.outDir, 'packages');
 
-  return Future.forEach(assets, (asset) {
+  return forEachPooledFuture(assets, (asset) {
     var id = asset.id;
     var dir = _firstDir(id.path);
     if (dir == null) return null;
@@ -273,8 +274,8 @@ Future _emitPackagesDir(BarbackOptions options) {
   // Copy all the files we didn't process
   var dirs = options.packageDirs;
 
-  return Future.forEach(_polymerPackageDependencies, (package) {
-    return Future.forEach(_listPackageDir(package, 'lib', options), (relpath) {
+  return forEachPooledFuture(_polymerPackageDependencies, (package) {
+    return forEachPooledFuture(_listPackageDir(package, 'lib', options), (relpath) {
       var inpath = path.join(dirs[package], relpath);
       var outpath = path.join(outPackages, package, relpath.substring(4));
       return _copyFile(inpath, outpath);