Revert "dart2js cps: Refactor and optimize string concatenations."

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

Index: pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
diff --git a/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart b/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
index eed6448075057df99c451595eb18a4e113df3063..6ac916253bb82579b3f95b90b9fbcd844ce5f9b0 100644
--- a/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
+++ b/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
@@ -557,6 +557,11 @@ class ParentVisitor extends RecursiveVisitor {
     node.arguments.forEach((Reference ref) => ref.parent = node);
   }
 
+  processConcatenateStrings(ConcatenateStrings node) {
+    node.continuation.parent = node;
+    node.arguments.forEach((Reference ref) => ref.parent = node);
+  }
+
   processBranch(Branch node) {
     node.condition.parent = node;
     node.trueContinuation.parent = node;