fix($injector): throw when factory $get method does not return a value

BREAKING CHANGE:

Previously, not returning a value would fail silently, and an application trying to inject the
value owuld inject an undefined value, quite possibly leading to a TypeError. Now, the application
will fail entirely, and a reason will be given.

Closes #4575

Author: caitp

docs/content/error/$injector/undef.ngdoc

@@ -0,0 +1,32 @@
+@ngdoc error
+@name $injector:undef
+@fullName Undefined Value
+@description
+
+This error results from registering a factory which does not return a value (or whose return value is undefined).
+
+The following is an example of a factory which will throw this error upon injection:
+
+```js
+angular.module("badModule", []).
+  factory("badFactory", function() {
+    doLotsOfThings();
+    butDontReturnAValue();
+  });
+```
+
+In order to prevent the error, return a value of some sort, such as an object which exposes an API for working
+with the injected object.
+
+```js
+angular.module("goodModule", []).
+  factory("goodFactory", function() {
+    doLotsOfThings();
+    butDontReturnAValue();
+
+    return {
+        doTheThing: function methodThatDoesAThing() {
+        }
+    };
+  });
+```

src/auto/injector.js

@@ -663,7 +663,17 @@ function createInjector(modulesToLoad, strictDi) {
     return providerCache[name + providerSuffix] = provider_;
   }
 
-  function factory(name, factoryFn) { return provider(name, { $get: factoryFn }); }
+  function enforceReturnValue(name, factory) {
+    return function enforcedReturnValue() {
+      var result = instanceInjector.invoke(factory);
+      if (isUndefined(result)) {
+        throw $injectorMinErr('undef', "Provider '{0}' must return a value from $get factory method.", name);
+      }
+      return result;
+    };
+  }
+
+  function factory(name, factoryFn) { return provider(name, { $get: enforceReturnValue(name, factoryFn) }); }
 
   function service(name, constructor) {
     return factory(name, ['$injector', function($injector) {

test/auto/injectorSpec.js

@@ -983,4 +983,14 @@ describe('strict-di injector', function() {
       }).toThrowMinErr('$injector', 'strictdi');
     });
   });
+
+
+  it('should throw if factory does not return a value', function() {
+    module(function($provide) {
+      $provide.factory('$test', function() {});
+    });
+    expect(function() {
+      inject(function($test) {});
+    }).toThrowMinErr('$injector', 'undef');
+  });
 });