add with strings bug fix

frog/corejs.dart

 // Copyright (c) 2011, 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.
 
 /**
  * Generates JS helpers for dart:core. This used to be in a file "core.js".
  * Having them in Dart code means we can easily control which are generated.
  */
 // TODO(jmesserly): one idea to make this cleaner: put these as private "native"
 // methods somewhere in a library that we import. This would be rather elegant
@@ skipping 65 matching lines @@
 }""";
         break;
 
       case '\$negate':
         code = @"""
 function $negate(x) {
   return (typeof(x) == 'number') ? -x : x.$negate();
 }""";
         break;
 
+      // This relies on JS's string "+" to match Dart's.
       case '\$add':
         code = @"""
 function $add(x, y) {
   return ((typeof(x) == 'number' && typeof(y) == 'number') ||
-          (typeof(x) == 'string' && typeof(y) == 'string'))
+          (typeof(x) == 'string'))
     ? x + y : x.$add(y);
 }""";
         break;
 
       case '\$truncdiv':
         useThrow = true;
         code = @"""
 function $truncdiv(x, y) {
   if (typeof(x) == 'number' && typeof(y) == 'number') {
     if (y == 0) $throw(new IntegerDivisionByZeroException());
@@ skipping 333 matching lines @@
         w.writeln(@"function $wrap_call$1(fn) { return fn; }");
       }
     }
 
     // Write operator helpers
     for (var opImpl in orderValuesByKeys(_usedOperators)) {
       w.writeln(opImpl);
     }
   }
 }