dart2js cps: Refactor and optimize string concatenations.

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

Index: pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
diff --git a/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart b/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
index 1fd6125ddab443b97f3bf2fb6653bd5d8d286059..f9f35de3fe4f87437e11cf7c01a9fc89e7e3d409 100644
--- a/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
+++ b/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
@@ -364,12 +364,6 @@ class Builder implements cps_ir.Visitor<Node> {
     return continueWithExpression(node.continuation, invoke);
   }
 
-  Statement visitConcatenateStrings(cps_ir.ConcatenateStrings node) {
-    List<Expression> arguments = translateArguments(node.arguments);
-    Expression concat = new ConcatenateStrings(arguments);
-    return continueWithExpression(node.continuation, concat);
-  }
-
   Statement visitThrow(cps_ir.Throw node) {
     Expression value = getVariableUse(node.value);
     return new Throw(value);
@@ -499,7 +493,7 @@ class Builder implements cps_ir.Visitor<Node> {
   }
 
   Expression visitConstant(cps_ir.Constant node) {
-    return new Constant(node.expression, node.value);
+    return new Constant(node.value);
   }
 
   Expression visitLiteralList(cps_ir.LiteralList node) {