feat(di): provide two ways to create an injector, resolved and unresolved

Add two factory static functions to Injector: resolveAndCreate and
fromResolvedBindings.

We want to avoid resolution and flattening every time we create a new
injector. This commit allows the user to cache resolved bindings and
reuse them.

modules/benchpress/test/sampler_spec.js

@@ -68,7 +68,7 @@ export function main() {
         ListWrapper.push(bindings, bind(Options.FORCE_GC).toValue(forceGc));
       }
 
-      sampler = new Injector(bindings).get(Sampler);
+      sampler = Injector.resolveAndCreate(bindings).get(Sampler);
     }
 
     it('should call the prepare and execute callbacks using WebDriverAdapter.waitFor', inject([AsyncTestCompleter], (async) => {