dart2js: Support new const operations on strings.

pkg/compiler/lib/src/js_backend/constant_system_javascript.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 js_backend;
 
 const JAVA_SCRIPT_CONSTANT_SYSTEM = const JavaScriptConstantSystem();
 
 class JavaScriptBitNotOperation extends BitNotOperation {
   const JavaScriptBitNotOperation();
@@ skipping 79 matching lines @@
     if (constant.isInt) {
       IntConstantValue intConstant = constant;
       if (intConstant.primitiveValue == 0) {
         return JAVA_SCRIPT_CONSTANT_SYSTEM.createDouble(-0.0);
       }
     }
     return dartNegateOperation.fold(constant);
   }
 }
 
+class JavaScriptAddOperation implements BinaryOperation {
+  final _addOperation = const AddOperation();
+  String get name => _addOperation.name;
+
+  const JavaScriptAddOperation();
+
+  ConstantValue fold(ConstantValue left, ConstantValue right) {
+    ConstantValue result = _addOperation.fold(left, right);
+    if (result != null && result.isNum) {
+      return JAVA_SCRIPT_CONSTANT_SYSTEM.convertToJavaScriptConstant(result);
+    }
+    return result;
+  }
+
+  apply(left, right) => _addOperation.apply(left, right);
+}
+
 class JavaScriptBinaryArithmeticOperation implements BinaryOperation {
   final BinaryOperation dartArithmeticOperation;
 
   const JavaScriptBinaryArithmeticOperation(this.dartArithmeticOperation);
 
   String get name => dartArithmeticOperation.name;
 
   ConstantValue fold(ConstantValue left, ConstantValue right) {
     ConstantValue result = dartArithmeticOperation.fold(left, right);
     if (result == null) return result;
@@ skipping 29 matching lines @@
 }
 
 /**
  * Constant system following the semantics for Dart code that has been
  * compiled to JavaScript.
  */
 class JavaScriptConstantSystem extends ConstantSystem {
   final int BITS31 = 0x8FFFFFFF;
   final int BITS32 = 0xFFFFFFFF;
 
-  final add = const JavaScriptBinaryArithmeticOperation(const AddOperation());
+  final add = const JavaScriptAddOperation();
   final bitAnd = const JavaScriptBinaryBitOperation(const BitAndOperation());
   final bitNot = const JavaScriptBitNotOperation();
   final bitOr = const JavaScriptBinaryBitOperation(const BitOrOperation());
   final bitXor = const JavaScriptBinaryBitOperation(const BitXorOperation());
   final booleanAnd = const BooleanAndOperation();
   final booleanOr = const BooleanOrOperation();
   final divide =
       const JavaScriptBinaryArithmeticOperation(const DivideOperation());
   final equal = const EqualsOperation();
   final greaterEqual = const GreaterEqualOperation();
@@ skipping 176 matching lines @@
       result.add(keyList);
     } else {
       // Add the keys individually to avoid generating an unused list constant
       // for the keys.
       result.addAll(keys);
     }
     result.addAll(values);
     return result;
   }
 }