Don't compile functions in a context the caller doesn't have access to

src/generator.js

Index: src/generator.js
diff --git a/src/generator.js b/src/generator.js
index 56579c59d40b65bc04356619240142b242536db5..a249a18f8517c1890289584c940b3d2dfd249234 100644
--- a/src/generator.js
+++ b/src/generator.js
@@ -80,7 +80,9 @@ function GeneratorFunctionConstructor(arg1) {  // length == 1
   var global_proxy = %GlobalProxy(GeneratorFunctionConstructor);
   // Compile the string in the constructor and not a helper so that errors
   // appear to come from here.
-  var f = %_CallFunction(global_proxy, %CompileString(source, true));
+  var f = %CompileString(source, true);
+  if (!IS_FUNCTION(f)) return f;
+  f = %_CallFunction(global_proxy, f);
   %FunctionMarkNameShouldPrintAsAnonymous(f);
   return f;
 }