Move pub/barback's Pool class into its own package.

pkg/barback/lib/src/utils/file_pool.dart

Index: pkg/barback/lib/src/utils/file_pool.dart
diff --git a/pkg/barback/lib/src/utils/file_pool.dart b/pkg/barback/lib/src/utils/file_pool.dart
index 6bea4a4631246d8e2fe05cb6f1e92b1e2352f8b3..6a233379daa786e96ee4ad2deba3c4704516b030 100644
--- a/pkg/barback/lib/src/utils/file_pool.dart
+++ b/pkg/barback/lib/src/utils/file_pool.dart
@@ -8,10 +8,10 @@ import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
+import 'package:pool/pool.dart';

[Bob Nystrom, 2014/07/17 22:09:14]

How about giving this a bit more specific name. Maybe "resource_pool"?

[nweiz, 2014/07/17 22:56:55]

I can if you want, but I think in a programming context it's reasonably clear
that "pool" refers to resources. It doesn't seem worthwhile to make people type
longer names when using the package if it's not disambiguating between things
they're likely to mix up.

Also I don't really like the idea of juxtaposing "ResourcePool" and
"PoolResource" classes.

[Bob Nystrom, 2014/07/21 17:47:36]

Maybe it's just my background, but I'd actually expect it to be an object or
memory pool.
Yeah, that would be weird. Just "pool" SGTM I guess.

 import 'package:stack_trace/stack_trace.dart';
 
 import '../utils.dart';
-import 'pool.dart';
 
 /// Manages a pool of files that are opened for reading to cope with maximum
 /// file descriptor limits.