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/dom/generic_browser_adapter.ts

@@ -23,7 +23,7 @@ export abstract class GenericBrowserDomAdapter extends DomAdapter {
           }
         }
       }
-      var transEndEventNames = {
+      var transEndEventNames: {[key: string]: string} = {
         WebkitTransition: 'webkitTransitionEnd',
         MozTransition: 'transitionend',
         OTransition: 'oTransitionEnd otransitionend',

modules/angular2/src/core/dom/parse5_adapter.ts

@@ -17,7 +17,7 @@ import {
 import {BaseException, WrappedException} from 'angular2/src/core/facade/exceptions';
 import {SelectorMatcher, CssSelector} from 'angular2/src/core/compiler/selector';
 
-var _attrToPropMap = {
+var _attrToPropMap: {[key: string]: string} = {
   'class': 'className',
   'innerHtml': 'innerHTML',
   'readonly': 'readOnly',