fix(di): throw if a token uses more than 20 dependencies.

Fixes #6690
Closes #6869

modules/angular2/test/core/di/injector_spec.ts

@@ -189,6 +189,49 @@ export function main() {
         expect(car.engine).toBeAnInstanceOf(Engine);
       });
 
+      it('should throw when using a factory with more than 20 dependencies', () => {
+        function factoryWithTooManyArgs() { return new Car(null); }
+
+        var injector = createInjector([
+          Engine,
+          provide(Car,
+                  {
+                    useFactory: factoryWithTooManyArgs,
+                    deps: [
+                      Engine,
+                      Engine,
+                      Engine,
+                      Engine,
+                      Engine,
+                      Engine,
+                      Engine,
+                      Engine,
+                      Engine,
+                      Engine,
+                      Engine,
+                      Engine,
+                      Engine,
+                      Engine,
+                      Engine,
+                      Engine,
+                      Engine,
+                      Engine,
+                      Engine,
+                      Engine,
+                      Engine
+                    ]
+                  })
+        ]);
+
+        try {
+          injector.get(Car);
+          throw "Must throw";
+        } catch (e) {
+          expect(e.message)
+              .toContain(`Cannot instantiate 'Car' because it has more than 20 dependencies`);
+        }
+      });
+
       it('should supporting provider to null', () => {
         var injector = createInjector([provide(Engine, {useValue: null})]);
         var engine = injector.get(Engine);