Propagate pseudo-constants expressions separately in tree rewriter.

pkg/compiler/lib/src/tree_ir/tree_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 tree_ir_tracer;
 
 import 'dart:async' show EventSink;
 import '../tracer.dart';
 import 'tree_ir_nodes.dart';
 import 'optimization/optimization.dart';
@@ skipping 220 matching lines @@
     add("$bci $uses $name $contents <|@\n");
   }
 
   visitLabeledStatement(LabeledStatement node) {
     // These do not get added to a block's list of statements.
   }
 
   visitAssign(Assign node) {
     String name = names.varName(node.variable);
     String rhs = expr(node.definition);
-    String extra = node.hasExactlyOneUse ? "[single-use]" : "";
+    Variable v = node.variable;
+    String extra = "(r=${v.readCount}, w=${v.writeCount})";
     printStatement(null, "assign $name = $rhs $extra");
   }
 
   visitReturn(Return node) {
     printStatement(null, "return ${expr(node.value)}");
   }
 
   visitBreak(Break node) {
     printStatement(null, "break ${collector.breakTargets[node.target].name}");
   }
@@ skipping 221 matching lines @@
       String prefix = v.element == null ? 'v' : '${v.element.name}_';
       while (name == null || _usedNames.contains(name)) {
         name = "$prefix${_counter++}";
       }
       _names[v] = name;
       _usedNames.add(name);
     }
     return name;
   }
 }