Remove support for String operator + from dart2js.

dart/lib/compiler/implementation/operations.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.
 
 interface Operation {
   final SourceString name;
   bool isUserDefinable();
 }
 
 interface UnaryOperation extends Operation {
@@ skipping 210 matching lines @@
   const AddOperation();
   Constant fold(Constant left, Constant right) {
     if (left.isInt() && right.isInt()) {
       IntConstant leftInt = left;
       IntConstant rightInt = right;
       return new IntConstant(leftInt.value + rightInt.value);
     } else if (left.isNum() && right.isNum()) {
       NumConstant leftNum = left;
       NumConstant rightNum = right;
       return new DoubleConstant(leftNum.value + rightNum.value);
-    } else if (left.isString() && !right.isObject()) {
-      PrimitiveConstant primitiveRight = right;
-      DartString rightDartString = primitiveRight.toDartString();
-      StringConstant leftString = left;
-      if (rightDartString.isEmpty()) {
-        return left;
-      } else if (leftString.value.isEmpty()) {
-        return new StringConstant(rightDartString, leftString.node);
-      } else {
-        DartString concatenated =
-            new ConsDartString(leftString.value, rightDartString);
-        return new StringConstant(concatenated, leftString.node);
-      }
     } else {
       return null;
     }
   }
 }
 
 class RelationalNumOperation implements BinaryOperation {
   bool isUserDefinable() => true;
   const RelationalNumOperation();
   Constant fold(Constant left, Constant right) {
@@ skipping 59 matching lines @@
   bool isUserDefinable() => false;
   const IdentityOperation();
   Constant fold(Constant left, Constant right) {
     // In order to preserve runtime semantics which says that NaN !== NaN don't
     // constant fold NaN === NaN. Otherwise the output depends on inlined
     // variables and other optimizations.
     if (left.isNaN() && right.isNaN()) return null;
     return new BoolConstant(left == right);
   }
 }