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/core/zone/ng_zone.ts

@@ -251,7 +251,7 @@ export class NgZone {
     var errorHandling;
 
     if (enableLongStackTrace) {
-      errorHandling = StringMapWrapper.merge(Zone.longStackTraceZone,
+      errorHandling = StringMapWrapper.merge(<any>Zone.longStackTraceZone,
                                              {onError: function(e) { ngZone._onError(this, e); }});
     } else {
       errorHandling = {onError: function(e) { ngZone._onError(this, e); }};