Move shared optimizations to the tree_ir directory.

pkg/compiler/lib/src/tree_ir/optimization/loop_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 loop_rewriter;
-
-import '../tree_ir/tree_ir_nodes.dart';
+part of tree_ir.optimization;
 
 /// Rewrites [WhileTrue] statements with an [If] body into a [WhileCondition],
 /// in situations where only one of the branches contains a [Continue] to the
 /// loop. Schematically:
 ///
 ///   L:
 ///   while (true) {
 ///     if (E) {
 ///       S1  (has references to L)
 ///     } else {
 ///       S2  (has no references to L)
 ///     }
 ///   }
 ///     ==>
 ///   L:
 ///   while (E) {
 ///     S1
 ///   };
 ///   S2
 ///
 /// A similar transformation is used when S2 occurs in the 'then' position.
 ///
 /// Note that the above pattern needs no iteration since nested ifs
 /// have been collapsed previously in the [StatementRewriter] phase.
-class LoopRewriter extends RecursiveVisitor {
+class LoopRewriter extends RecursiveVisitor implements Pass {
 
   Set<Label> usedContinueLabels = new Set<Label>();
 
   void rewrite(FunctionDefinition function) {
     if (function.isAbstract) return;
 
     function.body = visitStatement(function.body);
   }
 
   Statement visitLabeledStatement(LabeledStatement node) {
@@ skipping 82 matching lines @@
   Statement visitFunctionDeclaration(FunctionDeclaration node) {
     new LoopRewriter().rewrite(node.definition);
     node.next = visitStatement(node.next);
     return node;
   }
 
   void visitFunctionExpression(FunctionExpression node) {
     new LoopRewriter().rewrite(node.definition);
   }
 }