Move shared optimizations to the tree_ir directory.

pkg/compiler/lib/src/tree_ir/optimization/copy_propagator.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 copy_propagator;
-
-import '../elements/elements.dart';
-import '../tree_ir/tree_ir_nodes.dart';
+part of tree_ir.optimization;
 
 /// Eliminates moving assignments, such as w := v, by assigning directly to w
 /// at the definition of v.
 ///
 /// This compensates for suboptimal register allocation, and merges closure
 /// variables with local temporaries that were left behind when translating
 /// out of CPS (where closure variables live in a separate space).
-class CopyPropagator extends RecursiveVisitor {
+class CopyPropagator extends RecursiveVisitor implements Pass {
 
   /// After visitStatement returns, [move] maps a variable v to an
   /// assignment A of form w := v, under the following conditions:
   /// - there are no uses of w before A
   /// - A is the only use of v
   Map<Variable, Assign> move = <Variable, Assign>{};
 
   /// Like [move], except w is the key instead of v.
   Map<Variable, Assign> inverseMove = <Variable, Assign>{};
 
@@ skipping 163 matching lines @@
     node.next = visitStatement(node.next);
     visitExpression(node.expression);
     return node;
   }
 
   void visitFunctionExpression(FunctionExpression node) {
     new CopyPropagator().rewrite(node.definition);
   }
 
 }