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

pkg/compiler/lib/src/ssa/value_set.dart

 // Copyright (c) 2012, 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 ssa;
+import '../universe/side_effects.dart' show SideEffects;
+
+import 'nodes.dart';
+
 
 class ValueSet {
   int size = 0;
   List<HInstruction> table;
   ValueSetNode collisions;
   ValueSet() : table = new List<HInstruction>(8);
 
   bool get isEmpty => size == 0;
   int get length => size;
 
@@ skipping 132 matching lines @@
   }
 }
 
 class ValueSetNode {
   final HInstruction value;
   final int hash;
   int get hashCode => hash;
   ValueSetNode next;
   ValueSetNode(this.value, this.hash, this.next);
 }