Move shared optimizations to the tree_ir directory.

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

Index: pkg/compiler/lib/src/tree_ir/optimization/logical_rewriter.dart
diff --git a/pkg/compiler/lib/src/dart_backend/logical_rewriter.dart b/pkg/compiler/lib/src/tree_ir/optimization/logical_rewriter.dart
similarity index 98%
rename from pkg/compiler/lib/src/dart_backend/logical_rewriter.dart
rename to pkg/compiler/lib/src/tree_ir/optimization/logical_rewriter.dart
index 41727739b33dc18e92ddb5f3a9e242b9a3d7fc81..f4b1a4b50fe87771404836010c7ef0ba972acbab 100644
--- a/pkg/compiler/lib/src/dart_backend/logical_rewriter.dart
+++ b/pkg/compiler/lib/src/tree_ir/optimization/logical_rewriter.dart
@@ -2,10 +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 logical_rewriter;
-
-import '../constants/values.dart' as values;
-import '../tree_ir/tree_ir_nodes.dart';
+part of tree_ir.optimization;
 
 /// Rewrites logical expressions to be more compact in the Tree IR.
 ///
@@ -57,7 +54,7 @@ import '../tree_ir/tree_ir_nodes.dart';
 ///   x && !!y          (!!y validated by [isBooleanValued])
 ///   x && y            (double negation removed by [putInBooleanContext])
 ///
-class LogicalRewriter extends Visitor<Statement, Expression> {
+class LogicalRewriter extends Visitor<Statement, Expression> implements Pass {
 
   /// Statement to be executed next by natural fallthrough. Although fallthrough
   /// is not introduced in this phase, we need to reason about fallthrough when