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

lib/src/js/template.dart

 // Copyright (c) 2014, 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 TemplateManager {
   Map<String, Template> expressionTemplates = new Map<String, Template>();
   Map<String, Template> statementTemplates = new Map<String, Template>();
 
@@ skipping 100 matching lines @@
           arguments.keys.where((name) => !holeNames.contains(name)).join(", ");
       throw "Template arguments has unused mappings: $unusedNames";
     }
     if (!holeNames.every((String name) => arguments.containsKey(name))) {
       String notFound =
           holeNames.where((name) => !arguments.containsKey(name)).join(", ");
       throw "Template arguments is missing mappings for: $notFound";
     }
     return instantiator(arguments);
   }
+
+  /// Like [instantiate] but works with free variables.
+  Node safeCreate(Map arguments) {
+    if (holeNames.isEmpty) return ast;
+    return instantiator(new Map.fromIterable(holeNames, value: (name) {
+      var a = arguments[name];
+      return a != null ? a : new InterpolatedExpression(name);
+    }));
+  }
 }
 
 /**
  * An Instantiator is a Function that generates a JS AST tree or List of
  * trees. [arguments] is a List for positional templates, or Map for
  * named templates.
  */
 typedef Instantiator(var arguments);
 
 
@@ skipping 700 matching lines @@
     if (count != before) containsInterpolatedNode.add(node);
     return null;
   }
 
   visitInterpolatedNode(InterpolatedNode node) {
     containsInterpolatedNode.add(node);
     if (node.isNamed) holeNames.add(node.nameOrPosition);
     ++count;
   }
 }