Move shared optimizations to the tree_ir directory.

pkg/compiler/lib/src/tree_ir/optimization/logical_rewriter.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.
 
-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.
 ///
 /// In this class an expression is said to occur in "boolean context" if
 /// its result is immediately applied to boolean conversion.
 ///
 /// IF STATEMENTS:
 ///
 /// We apply the following two rules to [If] statements (see [visitIf]).
 ///
@@ skipping 31 matching lines @@
 ///
 ///   x ? y : false  ==>  x && y   (if y is known to be a boolean)
 ///
 /// The following sequence of rewrites demonstrates the merit of these rules:
 ///
 ///   x ? (y ? true : false) : false
 ///   x ? !!y : false   (double negation introduced by [toBoolean])
 ///   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
   /// evaluating the benefit of swapping the branches of an [If].
   Statement fallthrough;
 
   void rewrite(FunctionDefinition definition) {
     if (definition.isAbstract) return;
 
     definition.body = visitStatement(definition.body);
@@ skipping 373 matching lines @@
   }
 
   /// Destructively updates each entry of [l] with the result of visiting it.
   void _rewriteList(List<Expression> l) {
     for (int i = 0; i < l.length; i++) {
       l[i] = visitExpression(l[i]);
     }
   }
 }