Propagate pseudo-constants expressions separately in tree rewriter.

pkg/compiler/lib/src/cps_ir/cps_ir_tracer.dart

Index: pkg/compiler/lib/src/cps_ir/cps_ir_tracer.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_tracer.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_tracer.dart
index 2fc8597df24155e9dfac334c7c8d856d368fac7e..bb50842032133a0117f4d2209d06dd2d235579ff 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_tracer.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_tracer.dart
@@ -466,6 +466,10 @@ class BlockCollector extends cps_ir.Visitor {
     visit(exp.body);
   }
 
+  visitLetMutable(cps_ir.LetMutable exp) {
+    visit(exp.body);
+  }
+
   visitSetField(cps_ir.SetField exp) {
     visit(exp.body);
   }