dart2js: Support new const operations on strings.

sdk/lib/_internal/compiler/implementation/constant_system_dart.dart

 // Copyright (c) 2012, 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.
 
 part of dart2js;
 
 const DART_CONSTANT_SYSTEM = const DartConstantSystem();
 
 class BitNotOperation implements UnaryOperation {
   final String name = '~';
@@ skipping 213 matching lines @@
     if (left.isInt && right.isInt) {
       IntConstant leftInt = left;
       IntConstant rightInt = right;
       int result = leftInt.value + rightInt.value;
       return DART_CONSTANT_SYSTEM.createInt(result);
     } else if (left.isNum && right.isNum) {
       NumConstant leftNum = left;
       NumConstant rightNum = right;
       double result = leftNum.value + rightNum.value;
       return DART_CONSTANT_SYSTEM.createDouble(result);
+    } else if (left.isString && right.isString) {
+      StringConstant leftString = left;
+      StringConstant rightString = right;
+      DartString result =
+          new DartString.concat(leftString.value, rightString.value);
+      return DART_CONSTANT_SYSTEM.createString(result);
     } else {
       return null;
     }
   }
   apply(left, right) => left + right;
 }
 
 abstract class RelationalNumOperation implements BinaryOperation {
   const RelationalNumOperation();
   Constant fold(Constant left, Constant right) {
@@ skipping 116 matching lines @@
   bool isInt(Constant constant) => constant.isInt;
   bool isDouble(Constant constant) => constant.isDouble;
   bool isString(Constant constant) => constant.isString;
   bool isBool(Constant constant) => constant.isBool;
   bool isNull(Constant constant) => constant.isNull;
 
   bool isSubtype(Compiler compiler, DartType s, DartType t) {
     return compiler.types.isSubtype(s, t);
   }
 }