dart2js: Support new const operations on strings.

pkg/compiler/lib/src/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 215 matching lines @@
     if (left.isInt && right.isInt) {
       IntConstantValue leftInt = left;
       IntConstantValue rightInt = right;
       int result = leftInt.primitiveValue + rightInt.primitiveValue;
       return DART_CONSTANT_SYSTEM.createInt(result);
     } else if (left.isNum && right.isNum) {
       NumConstantValue leftNum = left;
       NumConstantValue rightNum = right;
       double result = leftNum.primitiveValue + rightNum.primitiveValue;
       return DART_CONSTANT_SYSTEM.createDouble(result);
+    } else if (left.isString && right.isString) {
+      StringConstantValue leftString = left;
+      StringConstantValue rightString = right;
+      DartString result = new DartString.concat(leftString.primitiveValue,
+                                                rightString.primitiveValue);
+      return DART_CONSTANT_SYSTEM.createString(result);
     } else {
       return null;
     }
   }
   apply(left, right) => left + right;
 }
 
 abstract class RelationalNumOperation implements BinaryOperation {
   const RelationalNumOperation();
   ConstantValue fold(ConstantValue left, ConstantValue right) {
@@ skipping 153 matching lines @@
   bool isInt(ConstantValue constant) => constant.isInt;
   bool isDouble(ConstantValue constant) => constant.isDouble;
   bool isString(ConstantValue constant) => constant.isString;
   bool isBool(ConstantValue constant) => constant.isBool;
   bool isNull(ConstantValue constant) => constant.isNull;
 
   bool isSubtype(DartTypes types, DartType s, DartType t) {
     return types.isSubtype(s, t);
   }
 }