Move shared optimizations to the tree_ir directory.

pkg/compiler/lib/src/tree_ir/optimization/statement_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 statement_rewriter;
-
-import '../tree_ir/tree_ir_nodes.dart';
+part of tree_ir.optimization;
 
 /**
  * Performs the following transformations on the tree:
  * - Assignment propagation
  * - If-to-conditional conversion
  * - Flatten nested ifs
  * - Break inlining
  * - Redirect breaks
  *
  * The above transformations all eliminate statements from the tree, and may
@@ skipping 68 matching lines @@
  * to their label are redirected.
  * For example, where 'jump' is either break or continue:
  *
  *   L0: {... break L0 ...}; jump L1
  *     ==>
  *   {... jump L1 ...}
  *
  * 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;
 
   /// Substitution map for labels. Any break to a label L should be substituted
   /// for a break to L' if L maps to L'.
   Map<Label, Jump> labelRedirects = <Label, Jump>{};
 
   /// Returns the redirect target of [label] or [label] itself if it should not
   /// be redirected.
@@ skipping 437 matching lines @@
   }
 
   Expression makeCondition(Expression e, bool polarity) {
     return polarity ? e : new Not(e);
   }
 
   Statement getBranch(If node, bool polarity) {
     return polarity ? node.thenStatement : node.elseStatement;
   }
 }