Propagate pseudo-constants expressions separately in tree rewriter.

pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart

Index: pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
diff --git a/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart b/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
index 2f7e5fe7e2e94fdfd1e17765259374304641d032..3181b59be3448c22e0a9d4ccfecf4ca6c5606f53 100644
--- a/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
+++ b/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
@@ -87,7 +87,9 @@ class Builder extends cps_ir.Visitor<Node> {
     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