Propagate pseudo-constants expressions separately in tree rewriter.

pkg/compiler/lib/src/tree_ir/tree_ir_builder.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_builder;
 
 import '../dart2jslib.dart' as dart2js;
 import '../dart_types.dart';
 import '../elements/elements.dart';
 import '../cps_ir/cps_ir_nodes.dart' as cps_ir;
@@ skipping 69 matching lines @@
     Variable variable = new Variable(currentElement, irVariable.hint);
     local2mutable[irVariable] = variable;
     return variable;
   }
 
   Variable getMutableVariableReference(
         cps_ir.Reference<cps_ir.MutableVariable> reference) {
     if (reference.definition.host != currentElement) {
       return parent.getMutableVariableReference(reference);
     }
-    return local2mutable[reference.definition];
+    Variable variable = local2mutable[reference.definition];
+    ++variable.readCount;
+    return variable;
   }
 
   /// Obtains the variable representing the given primitive. Returns null for
   /// primitives that have no reference and do not need a variable.
   Variable getVariable(cps_ir.Primitive primitive) {
     if (primitive.registerIndex == null) {
       return null; // variable is unused
     }
     List<Variable> variables = local2variables.putIfAbsent(primitive.hint,
         () => <Variable>[]);
@@ skipping 486 matching lines @@
     // visited.
     internalError(CURRENT_ELEMENT_SPANNABLE, 'Unexpected IR node: $node.');
     return null;
   }
 
   Expression visitIsTrue(cps_ir.IsTrue node) {
     return getVariableReference(node.value);
   }
 }