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

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

Index: pkg/compiler/lib/src/helpers/debug_collection.dart
diff --git a/pkg/compiler/lib/src/helpers/debug_collection.dart b/pkg/compiler/lib/src/helpers/debug_collection.dart
index 8ff4be2277a44a8e4122d9140cc71206cf016897..74a3d2ec34ef7e079c8d04c83b996aafaa3c4a4f 100644
--- a/pkg/compiler/lib/src/helpers/debug_collection.dart
+++ b/pkg/compiler/lib/src/helpers/debug_collection.dart
@@ -2,7 +2,6 @@
 // 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);
 
@@ -328,4 +327,4 @@ class CollectionTypeAsserter {
     throw '$name: $text$elementType'
           '${showObjects ? ' ($element)' : ''}';
   }
-}
+}