fix($injector): ensure $get method invoked with provider context

0d3b69a broke this by calling $get with an undefined
context, which in strict mode would be undefined. This fixes this by ensuring that the
provider is used as the context, as it was originally.

Closes angular#9511
Closes angular#9512

Author: caitp

src/auto/injector.js

@@ -664,7 +664,7 @@ function createInjector(modulesToLoad, strictDi) {
 
   function enforceReturnValue(name, factory) {
     return function enforcedReturnValue() {
-      var result = instanceInjector.invoke(factory);
+      var result = instanceInjector.invoke(factory, this, undefined, name);
       if (isUndefined(result)) {
         throw $injectorMinErr('undef', "Provider '{0}' must return a value from $get factory method.", name);
       }

test/auto/injectorSpec.js

@@ -993,4 +993,21 @@ describe('strict-di injector', function() {
       inject(function($test) {});
     }).toThrowMinErr('$injector', 'undef');
   });
+
+
+  it('should always use provider as `this` when invoking a factory', function() {
+    var called = false;
+    function factoryFn() {
+      called = true;
+      // jshint -W040
+      expect(typeof this.$get).toBe('function');
+      // jshint +W040
+      return this;
+    }
+    module(function($provide) {
+      $provide.factory('$test', factoryFn);
+    });
+    inject(function($test) {});
+    expect(called).toBe(true);
+  });
 });