chore(typings): remove StringMap

This was a poorly typed attempt to mimic TypeScript's index signatures,
which we can use instead.
This eliminates a very strange type that we were exposing to users, but
not re-exporting through our public API.

Fixes #4483

modules/angular2/src/router/router_link.ts

@@ -1,5 +1,5 @@
 import {Directive} from '../core/metadata';
-import {StringMap, StringMapWrapper} from 'angular2/src/core/facade/collection';
+import {StringMapWrapper} from 'angular2/src/core/facade/collection';
 
 import {Router} from './router';
 import {Location} from './location';