dart2js cps: Refactor and optimize string concatenations.

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

Index: pkg/compiler/lib/src/cps_ir/builtin_operator.dart
diff --git a/pkg/compiler/lib/src/cps_ir/builtin_operator.dart b/pkg/compiler/lib/src/cps_ir/builtin_operator.dart
index 0995e342cf047a1cece617ff67e4889119deee57..70fe1b6d949e84113af8de7d39a2e0ab8490a5dc 100644
--- a/pkg/compiler/lib/src/cps_ir/builtin_operator.dart
+++ b/pkg/compiler/lib/src/cps_ir/builtin_operator.dart
@@ -37,6 +37,13 @@ enum BuiltinOperator {
   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.
   ///