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

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

Index: pkg/compiler/lib/src/ssa/variable_allocator.dart
diff --git a/pkg/compiler/lib/src/ssa/variable_allocator.dart b/pkg/compiler/lib/src/ssa/variable_allocator.dart
index fb357050abae28ea942909fb4f3049d8d09ddaae..280a047e82e97c865fb361894b259e5b84d1fcc9 100644
--- a/pkg/compiler/lib/src/ssa/variable_allocator.dart
+++ b/pkg/compiler/lib/src/ssa/variable_allocator.dart
@@ -2,7 +2,11 @@
 // 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 '../common.dart';
+import '../compiler.dart' show Compiler;
+import '../js_backend/js_backend.dart';
+
+import 'nodes.dart';
 
 /**
  * The [LiveRange] class covers a range where an instruction is live.