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 4e6c64f497486cfa87f0870e3726cd65926a4bff..443e170e67e5c651c2f7c4539f69fb1de9b08c4f 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);
@@ -513,10 +506,6 @@ class BlockCollector implements cps_ir.Visitor {
     addEdgeToContinuation(exp.continuation);
   }
 
-  visitConcatenateStrings(cps_ir.ConcatenateStrings exp) {
-    addEdgeToContinuation(exp.continuation);
-  }
-
   visitThrow(cps_ir.Throw exp) {
   }