Cleanup 1: split parts into their own libraries, remove unused imports

pkg/compiler/lib/src/helpers/debug_collection.dart

 // Copyright (c) 2014, 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.
 
-part of dart2js.helpers;
 
 typedef void DebugCallback(String methodName, var arg1, var arg2);
 
 class DebugMap<K, V> implements Map<K, V> {
   final Map<K, V> map;
   DebugCallback indexSetCallBack;
   DebugCallback putIfAbsentCallBack;
 
   DebugMap(this.map, {DebugCallback addCallback}) {
     if (addCallback != null) {
@@ skipping 305 matching lines @@
   void call(String methodName, var element, _) {
     check(element, '$methodName: ');
   }
 
   void check(var element, [String text = '']) {
     String elementType = '${element.runtimeType}';
     if (runtimeTypes.contains(elementType)) return;
     throw '$name: $text$elementType'
           '${showObjects ? ' ($element)' : ''}';
   }
-}
+}