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/pipes/date_pipe.ts

@@ -81,7 +81,7 @@ var defaultLocale: string = 'en-US';
 @Pipe({name: 'date'})
 @Injectable()
 export class DatePipe implements PipeTransform {
-  static _ALIASES = {
+  static _ALIASES: {[key: string]: String} = {
     'medium': 'yMMMdjms',
     'short': 'yMdjm',
     'fullDate': 'yMMMMEEEEd',

modules/angular2/src/core/pipes/pipes.ts

@@ -14,7 +14,7 @@ import * as cd from 'angular2/src/core/change_detection/pipes';
 
 export class ProtoPipes {
   static fromBindings(bindings: PipeBinding[]): ProtoPipes {
-    var config = {};
+    var config: {[key: string]: PipeBinding} = {};
     bindings.forEach(b => config[b.name] = b);
     return new ProtoPipes(config);
   }