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

sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library pub.dart2js_transformer;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'package:analyzer/analyzer.dart';
 import 'package:barback/barback.dart';
 import 'package:path/path.dart' as path;
+import 'package:pool/pool.dart';
 import 'package:stack_trace/stack_trace.dart';
 
 import '../../../../compiler/compiler.dart' as compiler;
 import '../../../../compiler/implementation/dart2js.dart'
     show AbortLeg;
 import '../../../../compiler/implementation/source_file.dart';
 import '../barback.dart';
 import '../dart.dart' as dart;
-import '../pool.dart';
 import '../utils.dart';
 import 'asset_environment.dart';
 
 /// The set of all valid configuration options for this transformer.
 final _validOptions = new Set<String>.from([
   'commandLineOptions', 'checked', 'csp', 'minify', 'verbose', 'environment',
   'analyzeAll', 'suppressWarnings', 'suppressHints', 'suppressPackageWarnings',
   'terse'
 ]);
 
@@ skipping 373 matching lines @@
   }
 }
 
 /// An [EventSink] that discards all data. Provided to dart2js when we don't
 /// want an actual output.
 class NullSink<T> implements EventSink<T> {
   void add(T event) {}
   void addError(errorEvent, [StackTrace stackTrace]) {}
   void close() {}
 }