fix(typings): repair broken type-checking for StringMap

Note that the previous type of StringMap was overly permissive and didn't catch errors.
Also we have to explicitly type empty objects, which is explained here:
https://github.com/Microsoft/TypeScript/issues/5089

Closes #4487

modules/angular2/src/router/route_registry.ts

@@ -155,7 +155,7 @@ export class RouteRegistry {
     }
 
     var componentRecognizer = this._rules.get(parentComponent);
-    var auxInstructions = {};
+    var auxInstructions: {[key: string]: Instruction} = {};
 
     var promises = instruction.auxUrls.map((auxSegment: Url) => {
       var match = componentRecognizer.recognizeAuxiliary(auxSegment);