dart2js cps: Refactor and optimize string concatenations.

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

Index: pkg/compiler/lib/src/tree_ir/tree_ir_tracer.dart
diff --git a/pkg/compiler/lib/src/tree_ir/tree_ir_tracer.dart b/pkg/compiler/lib/src/tree_ir/tree_ir_tracer.dart
index fee1dd7adcb33d52b63659533e63a44f2bcabc96..7676806fe359b6758e99090486b68542584b8362 100644
--- a/pkg/compiler/lib/src/tree_ir/tree_ir_tracer.dart
+++ b/pkg/compiler/lib/src/tree_ir/tree_ir_tracer.dart
@@ -376,11 +376,6 @@ class SubexpressionVisitor extends ExpressionVisitor<String> {
     return "$keyword $callName($args)";
   }
 
-  String visitConcatenateStrings(ConcatenateStrings node) {
-    String args = node.arguments.map(visitExpression).join(', ');
-    return "concat [$args]";
-  }
-
   String visitLiteralList(LiteralList node) {
     String values = node.values.map(visitExpression).join(', ');
     return "list [$values]";