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/src/runner.js

@@ -50,7 +50,7 @@ export class Runner {
     if (isPresent(bindings)) {
       ListWrapper.push(sampleBindings, bindings);
     }
-    return new Injector(sampleBindings).asyncGet(Sampler)
+    return Injector.resolveAndCreate(sampleBindings).asyncGet(Sampler)
       .then( (sampler) => sampler.sample() );
   }
 }