restore arrow function bind this workaround

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 925 matching lines @@
     parameter.accept(visitor);
   }
   int get precedenceLevel => PRIMARY;
 }
 
 class This extends Expression {
   accept(NodeVisitor visitor) => visitor.visitThis(this);
   This _clone() => new This();
   int get precedenceLevel => PRIMARY;
   void visitChildren(NodeVisitor visitor) {}
+
+  static bool foundIn(Node node) {
+    _thisFinder.found = false;
+    node.accept(_thisFinder);
+    return _thisFinder.found;
+  }
 }
 
+final _thisFinder = new _ThisFinder();
+class _ThisFinder extends BaseVisitor {
+  bool found = false;
+  visitThis(This node) {
+    found = true;
+  }
+  visitNode(Node node) {
+    if (!found) super.visitNode(node);
+  }
+}
+
+
 // `super` is more restricted in the ES6 spec, but for simplicity we accept
 // it anywhere that `this` is accepted.
 class Super extends Expression {
   accept(NodeVisitor visitor) => visitor.visitSuper(this);
   Super _clone() => new Super();
   int get precedenceLevel => PRIMARY;
   void visitChildren(NodeVisitor visitor) {}
 }
 
 class NamedFunction extends Expression {
   final Identifier name;
   final Fun function;
 
   NamedFunction(this.name, this.function);
 
   accept(NodeVisitor visitor) => visitor.visitNamedFunction(this);
 
   void visitChildren(NodeVisitor visitor) {
     name.accept(visitor);
     function.accept(visitor);
   }
   NamedFunction _clone() => new NamedFunction(name, function);
 
-  int get precedenceLevel => CALL;
+  int get precedenceLevel => PRIMARY_LOW_PRECEDENCE;
 }
 
 abstract class FunctionExpression extends Expression {
   List<Parameter> get params;
   get body; // Expression or block
 }
 
 class Fun extends FunctionExpression {
   final List<Parameter> params;
   final Block body;
   /** Whether this is a JS generator (`function*`) that may contain `yield`. */
   final bool isGenerator;
 
   final AsyncModifier asyncModifier;
 
   Fun(this.params, this.body, {this.isGenerator: false,
       this.asyncModifier: const AsyncModifier.sync()});
 
   accept(NodeVisitor visitor) => visitor.visitFun(this);
 
   void visitChildren(NodeVisitor visitor) {
     for (Parameter param in params) param.accept(visitor);
     body.accept(visitor);
   }
 
   Fun _clone() => new Fun(params, body,
       isGenerator: isGenerator, asyncModifier: asyncModifier);
 
-  int get precedenceLevel => ASSIGNMENT;
+  int get precedenceLevel => PRIMARY_LOW_PRECEDENCE;
 }
 
 class ArrowFun extends FunctionExpression {
   final List<Parameter> params;
   final body; // Expression or Block
 
+  bool _closesOverThis; // lazy initialized
+
   ArrowFun(this.params, this.body);
 
   accept(NodeVisitor visitor) => visitor.visitArrowFun(this);
 
   void visitChildren(NodeVisitor visitor) {
     for (Parameter param in params) param.accept(visitor);
     body.accept(visitor);
   }
+  /// True if this function actually closes of `this`. We use this in some
+  /// situations to generate different code.
+  bool get closesOverThis {
+    if (_closesOverThis == null) _closesOverThis = This.foundIn(this);
+    return _closesOverThis;
+  }
+
+  int get precedenceLevel => PRIMARY_LOW_PRECEDENCE;
 
   ArrowFun _clone() => new ArrowFun(params, body);
-
-  /// 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;
 }
 
 /**
  * The Dart sync, sync*, async, and async* modifier.
  * See [DartYield].
  *
  * This is not used for JS functions.
  */
 class AsyncModifier {
   final bool isAsync;
@@ skipping 268 matching lines @@
   accept(NodeVisitor visitor) => visitor.visitClassExpression(this);
 
   void visitChildren(NodeVisitor visitor) {
     name.accept(visitor);
     if (heritage != null) heritage.accept(visitor);
     for (Method element in methods) element.accept(visitor);
   }
 
   ClassExpression _clone() => new ClassExpression(name, heritage, methods);
 
-  int get precedenceLevel => PRIMARY;
+  int get precedenceLevel => PRIMARY_LOW_PRECEDENCE;
 }
 
 class Method extends Property {
   final bool isGetter;
   final bool isSetter;
   final bool isStatic;
 
   Method(Expression name, Fun function,
       {this.isGetter: false, this.isSetter: false, this.isStatic: false})
       : super(name, function) {
@@ skipping 185 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);
 }