move => bind this workaround to js_ast

lib/src/js/nodes.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;
 
 abstract class NodeVisitor<T> {
   T visitProgram(Program node);
 
   T visitBlock(Block node);
@@ skipping 872 matching lines @@
 
   Fun _clone() => new Fun(params, body, asyncModifier: asyncModifier);
 
   int get precedenceLevel => CALL;
 }
 
 class ArrowFun extends FunctionExpression {
   final List<Identifier> params;
   final body; // Expression or Block
 
+  bool _bindThisWorkaround; // lazy initialized
+
   ArrowFun(this.params, this.body);
 
   accept(NodeVisitor visitor) => visitor.visitArrowFun(this);
 
   void visitChildren(NodeVisitor visitor) {
     for (Identifier param in params) param.accept(visitor);
     body.accept(visitor);
   }
 
   ArrowFun _clone() => new ArrowFun(params, body);
 
+  /// This is a workaround for V8 arrow function bindings being not yet
+  /// implemented. See <https://code.google.com/p/v8/issues/detail?id=2700>
+  bool get bindThisWorkaround {
+    if (_bindThisWorkaround != null) return _bindThisWorkaround;
+    var visitor = new _ThisFinder();
+    body.accept(visitor);
+    return _bindThisWorkaround = visitor.found;
+  }
+
   /// Ensure parens always get generated if necessary.
   // TODO(jmesserly): I'm not sure the printer is handling this correctly for
   // function() { ... } either.
-  int get precedenceLevel => ASSIGNMENT;
+  int get precedenceLevel => bindThisWorkaround ? CALL : ASSIGNMENT;
+}
+
+class _ThisFinder extends BaseVisitor {
+  bool found = false;
+  visitThis(This node) {
+    found = true;
+  }
+  visitNode(Node node) {
+    if (!found) super.visitNode(node);
+  }
 }
 
 class AsyncModifier {
   final bool isAsync;
   final bool isYielding;
   final String description;
 
   const AsyncModifier.sync()
       : isAsync = false,
         isYielding = false,
@@ skipping 424 matching lines @@
   final Expression expression;
 
   CommentExpression(this.comment, this.expression);
 
   int get precedenceLevel => PRIMARY;
   accept(NodeVisitor visitor) => visitor.visitCommentExpression(this);
   CommentExpression _clone() => new CommentExpression(comment, expression);
 
   void visitChildren(NodeVisitor visitor) => expression.accept(visitor);
 }