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

frog/minfrog

 #!/usr/bin/env node
 //  ********** Library dart:core **************
 //  ********** Natives dart:core **************
 /**
  * Generates a dynamic call stub for a function.
  * Our goal is to create a stub method like this on-the-fly:
  *   function($0, $1, capture) { return this($0, $1, true, capture); }
  *
  * This stub then replaces the dynamic one on Function, with one that is
  * specialized for that particular function, taking into account its default
@@ skipping 3556 matching lines @@
     var name = TokenKind.binaryMethodName(node.op.kind);
     if (node.op.kind == 49/*TokenKind.NE*/) {
       name = ':ne';
     }
     if (name == null) {
       $globals.world.internalError(('unimplemented binary op ' + node.op), node.span);
       return;
     }
     return x.invoke$4(this, name, node, new Arguments(null, [y]));
   }
+  else if ((assignKind != 0) && ((node.y instanceof BinaryExpression))) {
+    return this._visitAssign(assignKind, node.x, new ParenExpression(node.y, node.y.span), node, to$call$1(null), isVoid);
+  }
   else {
     return this._visitAssign(assignKind, node.x, node.y, node, to$call$1(null), isVoid);
   }
 }
 MethodGenerator.prototype._visitAssign = function(kind, xn, yn, position, captureOriginal, isVoid) {
   if (captureOriginal == null) {
     captureOriginal = (function (x) {
       return x;
     })
     ;
@@ skipping 9734 matching lines @@
   $globals._NO_COLOR = '\u001b[0m';
   $globals._RED_COLOR = '\u001b[31m';
 }
 var const$0 = new EnvMap()/*const EnvMap()*/;
 var const$2 = new EmptyQueueException()/*const EmptyQueueException()*/;
 var const$3 = new _DeletedKeySentinel()/*const _DeletedKeySentinel()*/;
 var const$8 = new NoMoreElementsException()/*const NoMoreElementsException()*/;
 var $globals = {};
 $static_init();
 main();