restore arrow function bind this workaround

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;
   final bool preferSemicolonToNewlineInMinifiedOutput;
 
-
   /// True to allow keywords in properties, such as `obj.var` or `obj.function`
   /// Modern JS engines support this.
   final bool allowKeywordsInProperties;
 
+  /// Workaround if `this` is not bound in arrow functions.
+  final bool arrowFnBindThisWorkaround;
+
   JavaScriptPrintingOptions(
       {this.shouldCompressOutput: false,
        this.minifyLocalVariables: false,
        this.preferSemicolonToNewlineInMinifiedOutput: false,
-       this.allowKeywordsInProperties: false});
+       this.allowKeywordsInProperties: false,
+       this.arrowFnBindThisWorkaround: false});
 }
 
 
 /// An environment in which JavaScript printing is done.  Provides emitting of
 /// text and pre- and post-visit callbacks.
 abstract class JavaScriptPrintingContext {
   /// Signals an error.  This should happen only for serious internal errors.
   void error(String message) { throw message; }
 
   /// Adds [string] to the output.
@@ skipping 508 matching lines @@
   }
 
   visitFunctionDeclaration(FunctionDeclaration declaration) {
     indent();
     functionOut(declaration.function, declaration.name);
     lineOut();
   }
 
   visitNestedExpression(Expression node, int requiredPrecedence,
                         {bool newInForInit, bool newAtStatementBegin}) {
+    int nodePrecedence = node.precedenceLevel;
+    if (options.arrowFnBindThisWorkaround) {
+      if (node is ArrowFun && node.closesOverThis) {
+        nodePrecedence = CALL;
+      }
+    }
     bool needsParentheses =
         // a - (b + c).
         (requiredPrecedence != EXPRESSION &&
-         node.precedenceLevel < requiredPrecedence) ||
+         nodePrecedence < requiredPrecedence) ||
         // for (a = (x in o); ... ; ... ) { ... }
         (newInForInit && node is Binary && node.op == "in") ||
         // (function() { ... })().
         // ({a: 2, b: 3}.toString()).
         (newAtStatementBegin && (node is NamedFunction ||
                                  node is FunctionExpression ||
                                  node is ObjectInitializer));
     if (needsParentheses) {
       inForInit = false;
       atStatementBegin = false;
@@ skipping 286 matching lines @@
 
   visitNamedFunction(NamedFunction namedFunction) {
     functionOut(namedFunction.function, namedFunction.name);
   }
 
   visitFun(Fun fun) {
     functionOut(fun, null);
   }
 
   visitArrowFun(ArrowFun fun) {
+    bool bindThis = options.arrowFnBindThisWorkaround && fun.closesOverThis;
+    if (bindThis) {
+      out("(");
+    }
     localNamer.enterScope(fun);
     if (fun.params.length == 1) {
       visitNestedExpression(fun.params.single, SPREAD,
           newInForInit: false, newAtStatementBegin: false);
     } else {
       out("(");
       visitCommaSeparated(fun.params, SPREAD,
           newInForInit: false, newAtStatementBegin: false);
       out(")");
     }
     spaceOut();
     out("=>");
     if (fun.body is Expression) {
       spaceOut();
       // Object initializers require parenthesis to disambiguate
       // AssignmentExpression from FunctionBody. See:
       // https://people.mozilla.org/~jorendorff/es6-draft.html#sec-arrow-function-definitions
       var needsParen = fun.body is ObjectInitializer;
       if (needsParen) out("(");
       visitNestedExpression(fun.body, ASSIGNMENT,
           newInForInit: false, newAtStatementBegin: false);
       if (needsParen) out(")");
     } else {
       blockBody(fun.body, needsSeparation: false, needsNewline: false);
     }
     localNamer.leaveScope();
+    if (bindThis) {
+      out(").bind(this)");
+    }
   }
 
   visitLiteralBool(LiteralBool node) {
     out(node.value ? "true" : "false");
   }
 
   visitLiteralString(LiteralString node) {
     out(node.value);
   }
 
@@ skipping 563 matching lines @@
     declare(node.name);
     node.function.accept(this);
   }
 
   visitClassExpression(ClassExpression node) {
     declare(node.name);
     if (node.heritage != null) node.heritage.accept(this);
     for (Method element in node.methods) element.accept(this);
   }
 }