When += (and similar operations) are converted to = + ,

frog/gen.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.
 
 /**
  * Top level generator object for writing code and keeping track of
  * dependencies.
  *
  * Should have two compilation models, but only one implemented so far.
  *
@@ skipping 1852 matching lines @@
       final y = visitValue(node.y);
       var name = TokenKind.binaryMethodName(node.op.kind);
       if (node.op.kind == TokenKind.NE) {
         name = ':ne';
       }
       if (name == null) {
         world.internalError('unimplemented binary op ${node.op}', node.span);
         return;
       }
       return x.invoke(this, name, node, new Arguments(null, [y]));
-    } else {
+    } else if ((assignKind != 0) && (node.y is BinaryExpression)){

[jimhug, 2011/12/15 17:58:10]

Final comment: Please add a TODO or unify with parenthesis adding code in
visitLiteralExpression.  I think that some of those MAY be relevant here as well
- but more significantly, this is hard enough that we should be only doing it in
one place.

[dgrove, 2011/12/15 20:55:51]

Done.

+      return _visitAssign(assignKind, node.x,
+          new ParenExpression(node.y, node.y.span), node, null, isVoid);
+    } else {  
       return _visitAssign(assignKind, node.x, node.y, node, null, isVoid);
     }
   }
 
   /**
    * Visits an assignment expression.
    * Note: captureOriginal can optionally capture the original value of the
    * left side. This is used by postfix expressions to ensure they return the
    * original value, before it has been modified.
    */
@@ skipping 654 matching lines @@
       result.add(new Value(world.varType, '\$$i', null, /*needsTemp:*/false));
     }
     for (int i = bareCount; i < length; i++) {
       var name = getName(i);
       if (name == null) name = '\$$i';
       result.add(new Value(world.varType, name, null, /*needsTemp:*/false));
     }
     return new Arguments(nodes, result);
   }
 }