Port fix in #11293 to correct file

Author: mhegazy

src/lib/es5.d.ts

@@ -244,6 +244,9 @@ interface Function {
       */
     bind(this: Function, thisArg: any, ...argArray: any[]): any;
 
+    /** Returns a string representation of a function. */
+    toString(): string;
+
     prototype: any;
     readonly length: number;
 

tests/cases/fourslash/functionTypes.ts

@@ -23,12 +23,13 @@
 verify.numberOfErrorsInCurrentFile(0);
 for (var i = 1; i <= 7; i++) {
     goTo.marker('' + i);
-    verify.memberListCount(7);
+    verify.memberListCount(8);
     verify.completionListContains('apply');
     verify.completionListContains('arguments');
     verify.completionListContains('bind');
     verify.completionListContains('call');
     verify.completionListContains('length');
     verify.completionListContains('caller');
     verify.completionListContains('prototype');
+    verify.completionListContains('toString');
 }