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

pkg/compiler/lib/src/use_unused_api.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.
 
 /// This file use methods that aren't used by dart2js.dart, but that we wish to
 /// keep anyway. This might be general API that isn't currently in use,
 /// debugging aids, or API only used for testing (see TODO below).
 
 library dart2js.use_unused_api;
 
@@ skipping 22 matching lines @@
 import 'io/source_map_builder.dart' as io;
 import 'js/js.dart' as js;
 import 'js_backend/js_backend.dart' as js_backend;
 import 'js_emitter/js_emitter.dart' as js_emitter;
 import 'js_emitter/full_emitter/emitter.dart' as full;
 import 'js_emitter/program_builder/program_builder.dart' as program_builder;
 import 'resolution/semantic_visitor.dart' as semantic_visitor;
 import 'resolution/operators.dart' as operators;
 import 'script.dart';
 import 'source_file_provider.dart' as source_file_provider;
-import 'ssa/ssa.dart' as ssa;
+import 'ssa/nodes.dart' as ssa;
 import 'tree/tree.dart' as tree;
 import 'util/util.dart' as util;
 import 'world.dart';
 
 import 'parser/partial_elements.dart' show
     PartialClassElement,
     PartialFunctionElement;
 
 class ElementVisitor extends elements_visitor.BaseElementVisitor {
   visitElement(e, a) {}
@@ skipping 282 matching lines @@
   noSuchMethod(inv) {}
 }
 
 useTreeVisitors() {
   new TreeVisitor1().visitExpression(null, null);
   new TreeVisitor1().visitStatement(null, null);
 }
 
 useDeferred([deferred.DeferredLoadTask task]) {
   task.dump();
-}
+}