dart2js cps: Refactor and optimize string concatenations.

pkg/compiler/lib/src/js_backend/codegen/codegen.dart

Index: pkg/compiler/lib/src/js_backend/codegen/codegen.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
index 78ccd5829e5cb30eaef2996fc59e5e308f9eba5c..a8cd15e9a70e641e3da87f399fc6a8180502cf9e 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
@@ -171,31 +171,6 @@ class CodeGenerator extends tree_ir.StatementVisitor
   }
 
   @override
-  js.Expression visitConcatenateStrings(tree_ir.ConcatenateStrings node) {
-    js.Expression addStrings(js.Expression left, js.Expression right) {
-      return new js.Binary('+', left, right);
-    }
-
-    js.Expression toString(tree_ir.Expression input) {
-      bool useDirectly = input is tree_ir.Constant &&
-          (input.value.isString ||
-           input.value.isInt ||
-           input.value.isBool);
-      js.Expression value = visitExpression(input);
-      if (useDirectly) {
-        return value;
-      } else {
-        Element convertToString = glue.getStringConversion();
-        registry.registerStaticUse(convertToString);
-        js.Expression access = glue.staticFunctionAccess(convertToString);
-        return (new js.Call(access, <js.Expression>[value]));
-      }
-    }
-
-    return node.arguments.map(toString).reduce(addStrings);
-  }
-
-  @override
   js.Expression visitConditional(tree_ir.Conditional node) {
     return new js.Conditional(
         visitExpression(node.condition),
@@ -724,6 +699,9 @@ class CodeGenerator extends tree_ir.StatementVisitor
         return new js.Binary('>', args[0], args[1]);
       case BuiltinOperator.NumGe:
         return new js.Binary('>=', args[0], args[1]);
+      case BuiltinOperator.StringConcat:
+        if (args.isEmpty) return js.string('');
+        return args.reduce((e1,e2) => new js.Binary('+', e1, e2));
       case BuiltinOperator.StrictEq:
         return new js.Binary('===', args[0], args[1]);
       case BuiltinOperator.StrictNeq: