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/test/test_lib/utils_spec.ts

@@ -187,7 +187,7 @@ export function main() {
       }
     ];
 
-    browsers.forEach((browser) => {
+    browsers.forEach((browser: {[key: string]: any}) => {
       it(`should detect ${StringMapWrapper.get(browser, 'name')}`, () => {
         var bd = new BrowserDetection(<string>StringMapWrapper.get(browser, 'ua'));
         expect(bd.isFirefox).toBe(StringMapWrapper.get(browser, 'isFirefox'));