fix($injector): ensure functions with overridden toString() are annotated properly

Closes #14361

src/auto/injector.js

@@ -71,7 +71,7 @@ var STRIP_COMMENTS = /((\/\/.*$)|(\/\*[\s\S]*?\*\/))/mg;
 var $injectorMinErr = minErr('$injector');
 
 function extractArgs(fn) {
-  var fnText = fn.toString().replace(STRIP_COMMENTS, ''),
+  var fnText = Function.prototype.toString.call(fn).replace(STRIP_COMMENTS, ''),
       args = fnText.match(ARROW_ARG) || fnText.match(FN_ARGS);
   return args;
 }

test/auto/injectorSpec.js

@@ -241,6 +241,12 @@ describe('injector', function() {
       expect($f_n0.$inject).toEqual(['$a_']);
     });
 
+    it('should handle functions with overridden toString', function() {
+      function fn(a) {}
+      fn.toString = function() { return 'fn'; };
+      expect(annotate(fn)).toEqual(['a']);
+      expect(fn.$inject).toEqual(['a']);
+    });
 
     it('should throw on non function arg', function() {
       expect(function() {