feat: remove MapWrapper.contains().

modules/angular2/src/http/headers.ts

@@ -53,7 +53,7 @@ export class Headers {
 
   get(header: string): string { return ListWrapper.first(this._headersMap.get(header)); }
 
-  has(header: string) { return MapWrapper.contains(this._headersMap, header); }
+  has(header: string) { return this._headersMap.has(header); }
 
   keys() { return MapWrapper.keys(this._headersMap); }
 

modules/angular2/src/http/url_search_params.ts

@@ -19,7 +19,7 @@ export class URLSearchParams {
   paramsMap: Map<string, List<string>>;
   constructor(public rawParams: string) { this.paramsMap = paramParser(rawParams); }
 
-  has(param: string): boolean { return MapWrapper.contains(this.paramsMap, param); }
+  has(param: string): boolean { return this.paramsMap.has(param); }
 
   get(param: string): string { return ListWrapper.first(this.paramsMap.get(param)); }