Move shared optimizations to the tree_ir directory.

pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart

Index: pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
diff --git a/pkg/compiler/lib/src/dart_backend/statement_rewriter.dart b/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
similarity index 99%
rename from pkg/compiler/lib/src/dart_backend/statement_rewriter.dart
rename to pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
index 6726ef9103010e65b7a6f69e00195f85c66a1243..405c93a75e8b7a5b107641d0866809dda17968a9 100644
--- a/pkg/compiler/lib/src/dart_backend/statement_rewriter.dart
+++ b/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
@@ -2,9 +2,7 @@
 // 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.
 
-library statement_rewriter;
-
-import '../tree_ir/tree_ir_nodes.dart';
+part of tree_ir.optimization;
 
 /**
  * Performs the following transformations on the tree:
@@ -93,7 +91,7 @@ import '../tree_ir/tree_ir_nodes.dart';
  * This may trigger a flattening of nested ifs in case the eliminated label
  * separated two ifs.
  */
-class StatementRewriter extends Visitor<Statement, Expression> {
+class StatementRewriter extends Visitor<Statement, Expression> implements Pass {
   // The binding environment.  The rightmost element of the list is the nearest
   // available enclosing binding.
   List<Assign> environment;