dart2js cps: Refactor and optimize string concatenations.

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 8b2b173d448ed519d167aa6752d618dbc29fc4c2..62f96cc67033c6c7a6845f5606330f8b53fa6336 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_tracer.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_tracer.dart
@@ -175,13 +175,6 @@ class IRTracer extends TracerUtil implements cps_ir.Visitor {
     printStmt(dummy, "InvokeConstructor $callName ($args) $kont");
   }
 
-  visitConcatenateStrings(cps_ir.ConcatenateStrings node) {
-    String dummy = names.name(node);
-    String args = node.arguments.map(formatReference).join(', ');
-    String kont = formatReference(node.continuation);
-    printStmt(dummy, "ConcatenateStrings ($args) $kont");
-  }
-
   visitThrow(cps_ir.Throw node) {
     String dummy = names.name(node);
     String value = formatReference(node.value);
@@ -503,10 +496,6 @@ class BlockCollector implements cps_ir.Visitor {
     addEdgeToContinuation(exp.continuation);
   }
 
-  visitConcatenateStrings(cps_ir.ConcatenateStrings exp) {
-    addEdgeToContinuation(exp.continuation);
-  }
-
   visitThrow(cps_ir.Throw exp) {
   }