Move shared optimizations to the tree_ir directory.

pkg/compiler/lib/src/js_backend/codegen/task.dart

 // Copyright (c) 2014, 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.
 
 /// Generate code using the cps-based IR pipeline.
 library code_generator_task;
 
 import 'glue.dart';
 import 'codegen.dart';
 
 import '../js_backend.dart';
 import '../../dart2jslib.dart';
 import '../../source_file.dart';
 import '../../cps_ir/cps_ir_nodes.dart' as cps;
 import '../../cps_ir/cps_ir_builder.dart';
 import '../../tree_ir/tree_ir_nodes.dart' as tree_ir;
 import '../../tree/tree.dart' as ast;
 import '../../scanner/scannerlib.dart' as scanner;
 import '../../elements/elements.dart';
 import '../../closure.dart';
 import '../../js/js.dart' as js;
 import '../../source_map_builder.dart';
 import '../../tree_ir/tree_ir_builder.dart' as tree_builder;
 import '../../dart_backend/backend_ast_emitter.dart' as backend_ast_emitter;
 import '../../cps_ir/optimizers.dart';
 import '../../tracer.dart';
-import '../../dart_backend/statement_rewriter.dart';
-import '../../dart_backend/copy_propagator.dart';
-import '../../dart_backend/loop_rewriter.dart';
-import '../../dart_backend/logical_rewriter.dart';
 import '../../js_backend/codegen/codegen.dart';
 import '../../ssa/ssa.dart' as ssa;
+import '../../tree_ir/optimization/optimization.dart';
 
 class CspFunctionCompiler implements FunctionCompiler {
   final IrBuilderTask irBuilderTask;
   final ConstantSystem constantSystem;
   final Compiler compiler;
 
   // TODO(karlklose,sigurm): remove and update dart-doc of [compile].
   final FunctionCompiler fallbackCompiler;
 
   // TODO(sigurdm): Assign this.
@@ skipping 153 matching lines @@
   SourceFile sourceFileOfElement(Element element) {
     return element.implementation.compilationUnit.script.file;
   }
 
   js.Fun buildJavaScriptFunction(FunctionElement element,
                                  List<js.Parameter> parameters,
                                  js.Block body) {
     return attachPosition(new js.Fun(parameters, body), element);
   }
 }