Propagate pseudo-constants expressions separately in tree rewriter.

pkg/compiler/lib/src/cps_ir/cps_ir_tracer.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 dart2js.ir_tracer;
 
 import 'dart:async' show EventSink;
 
 import 'cps_ir_nodes.dart' as cps_ir hide Function;
 import '../tracer.dart';
@@ skipping 448 matching lines @@
   }
 
   visitInvokeContinuation(cps_ir.InvokeContinuation exp) {
     addEdgeToContinuation(exp.continuation);
   }
 
   visitSetMutableVariable(cps_ir.SetMutableVariable exp) {
     visit(exp.body);
   }
 
+  visitLetMutable(cps_ir.LetMutable exp) {
+    visit(exp.body);
+  }
+
   visitSetField(cps_ir.SetField exp) {
     visit(exp.body);
   }
 
   visitDeclareFunction(cps_ir.DeclareFunction exp) {
     visit(exp.body);
   }
 
   visitBranch(cps_ir.Branch exp) {
     cps_ir.Continuation trueTarget = exp.trueContinuation.definition;
     if (!trueTarget.isReturnContinuation) {
       current_block.addEdgeTo(getBlock(trueTarget));
     }
     cps_ir.Continuation falseTarget = exp.falseContinuation.definition;
     if (!falseTarget.isReturnContinuation) {
       current_block.addEdgeTo(getBlock(falseTarget));
     }
   }
 
   visitContinuation(cps_ir.Continuation c) {
     var old_node = current_block;
     current_block = getBlock(c);
     visit(c.body);
     current_block = old_node;
   }
 }