Move constant systems into their own libraries.

pkg/compiler/lib/src/constants/constant_system.dart

Index: pkg/compiler/lib/src/constants/constant_system.dart
diff --git a/pkg/compiler/lib/src/constant_system.dart b/pkg/compiler/lib/src/constants/constant_system.dart
similarity index 95%
rename from pkg/compiler/lib/src/constant_system.dart
rename to pkg/compiler/lib/src/constants/constant_system.dart
index 6a1bfb0444d9603a6ed6946838f98d4cbdab3f92..cac7b93c00150cee12520d5c2bbb7f7d9c5192d4 100644
--- a/pkg/compiler/lib/src/constant_system.dart
+++ b/pkg/compiler/lib/src/constants/constant_system.dart
@@ -2,7 +2,12 @@
 // 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;
+library dart2js.constant_system;
+
+import '../dart_types.dart';
+import '../dart2jslib.dart' show Compiler;
+import '../tree/tree.dart' show DartString;
+import 'values.dart';
 
 abstract class Operation {
   String get name;