dart2js cps: Refactor and optimize string concatenations.

pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart

Index: pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
diff --git a/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart b/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
index b44ca6dfb819005b7d894da1825c9bae9cbd7607..8ef8fda6e8b1b93f5f6c9c5992f93563163f2081 100644
--- a/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
+++ b/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
@@ -388,11 +388,6 @@ class StatementRewriter extends Transformer implements Pass {
     return node;
   }
 
-  Expression visitConcatenateStrings(ConcatenateStrings node) {
-    _rewriteList(node.arguments);
-    return node;
-  }
-
   Expression visitConditional(Conditional node) {
     // Conditional expressions do not exist in the input, but they are
     // introduced by if-to-conditional conversion.