dart2js cps: Refactor and optimize string concatenations.

pkg/compiler/lib/src/cps_ir/builtin_operator.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 library builtin_operator;
 // This is shared by the CPS and Tree IRs.
 // Both cps_ir_nodes and tree_ir_nodes import and re-export this file.
 
 /// An operator supported natively in the CPS and Tree IRs using the
 /// `ApplyBuiltinOperator` instructions.
 ///
@@ skipping 19 matching lines @@
   NumSubtract,
   NumMultiply,
   NumAnd,
   NumOr,
   NumXor,
   NumLt,
   NumLe,
   NumGt,
   NumGe,
 
+  /// Concatenates any number of strings.
+  ///
+  /// Takes any number of arguments, and each argument must be a string.
+  ///
+  /// Returns the empty string if no arguments are given.
+  StringConcatenate,
+
   /// Returns true if the two arguments are the same value, and that value is
   /// not NaN, or if one argument is +0 and the other is -0.
   ///
   /// At most one of the arguments may be null.
   StrictEq,
 
   /// Negated version of [StrictEq]. Introduced by [LogicalRewriter] in Tree IR.
   StrictNeq,
 
   /// Returns true if the two arguments are both null or are the same string,
@@ skipping 30 matching lines @@
   /// Compiles to `Math.floor(x) === x`
   IsFloor,
 
   /// Returns true if the argument is an integer.
   ///
   /// The argument must be repeated 3 times.
   ///
   /// Compiles to `typeof x === 'number' && Math.floor(x) === x`
   IsNumberAndFloor,
 }