fix(browser_adapter.ts): baseElement.getAttribute

currently throwing an error
```error
Error during instantiation of LocationStrategy! (RouterLink -> Router
-> Location -> LocationStrategy).
ORIGINAL ERROR: TypeError: baseElement.attr is not a function
```

Closes #3214

modules/angular2/src/dom/browser_adapter.dart

@@ -423,7 +423,11 @@ class BrowserDomAdapter extends GenericBrowserDomAdapter {
       return null;
     }
     var baseUri = Uri.parse(href);
-    return baseUri.path;
+    return baseUri.path[0] == '/' ? baseUri.path : ('/' + baseUri.path);
+  }
+
+  resetBaseElement() {
+    baseElement = null;
   }
 
   String getUserAgent() {

modules/angular2/src/dom/browser_adapter.ts

@@ -297,6 +297,7 @@ export class BrowserDomAdapter extends GenericBrowserDomAdapter {
     }
     return relativePath(href);
   }
+  resetBaseElement(): void { baseElement = null; }
   getUserAgent(): string { return window.navigator.userAgent; }
   setData(element, name: string, value: string) { element.dataset[name] = value; }
   getData(element, name: string): string { return element.dataset[name]; }
@@ -313,7 +314,7 @@ function getBaseElementHref(): string {
       return null;
     }
   }
-  return baseElement.attr('href');
+  return baseElement.getAttribute('href');
 }
 
 // based on urlUtils.js in AngularJS 1

modules/angular2/src/dom/dom_adapter.ts

@@ -127,6 +127,7 @@ export class DomAdapter {
   getHistory(): History { throw _abstract(); }
   getLocation(): Location { throw _abstract(); }
   getBaseHref(): string { throw _abstract(); }
+  resetBaseElement(): void { throw _abstract(); }
   getUserAgent(): string { throw _abstract(); }
   setData(element, name: string, value: string) { throw _abstract(); }
   getData(element, name: string): string { throw _abstract(); }

modules/angular2/src/dom/html_adapter.dart

@@ -400,6 +400,10 @@ class Html5LibDomAdapter implements DomAdapter {
     throw 'not implemented';
   }
 
+  resetBaseElement() {
+    throw 'not implemented';
+  }
+
   String getUserAgent() {
     throw 'not implemented';
   }

modules/angular2/src/dom/parse5_adapter.ts

@@ -532,6 +532,8 @@ export class Parse5DomAdapter extends DomAdapter {
       return this.defaultDoc().body;
     }
   }
+  getBaseHref(): string { throw 'not implemented'; }
+  resetBaseElement(): void { throw 'not implemented'; }
   getHistory(): History { throw 'not implemented'; }
   getLocation(): Location { throw 'not implemented'; }
   getUserAgent(): string { return "Fake user agent"; }