implement opassign, fix bugs in pre/postfix, introduce a let* helper

lib/src/js/printer.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_ast;
 
 
 class JavaScriptPrintingOptions {
   final bool shouldCompressOutput;
   final bool minifyLocalVariables;
@@ skipping 820 matching lines @@
     out("(");
     if (fun.params != null) {
       visitCommaSeparated(fun.params, PRIMARY,
           newInForInit: false, newAtStatementBegin: false);
     }
     out(")");
     spaceOut();
     out("=>");
     if (fun.body is Expression) {
       spaceOut();
-      fun.body.accept(this);
+      visitNestedExpression(fun.body, ASSIGNMENT,
+          newInForInit: false, newAtStatementBegin: false);
     } else {
       blockBody(fun.body, needsSeparation: false, needsNewline: false);
     }
     localNamer.leaveScope();
     if (fun.bindThisWorkaround) {
       out(").bind(this)");
     }
   }
 
   visitLiteralBool(LiteralBool node) {
@@ skipping 523 matching lines @@
         codes.add(nthLetter((n ~/ nameSpaceSize) % LETTERS));
       }
       codes.add(charCodes.$0 + digit);
       newName = new String.fromCharCodes(codes);
     }
     assert(new RegExp(r'[a-zA-Z][a-zA-Z0-9]*').hasMatch(newName));
     maps.last[oldName] = newName;
     return newName;
   }
 }