fix(facade): use base element to get base href

Previously, calls to getBaseHref used document.baseURI, which defaults
to the current path in the absence of a base element in the document.
This leads to surprising behavior.

With this change, getBaseHref returns null when a base element is not
present in the document.

modules/angular2/src/dom/browser_adapter.dart

@@ -342,8 +342,11 @@ class BrowserDomAdapter extends GenericBrowserDomAdapter {
     return window.location;
   }
   getBaseHref() {
-    var uri = document.baseUri;
-    var baseUri = Uri.parse(uri);
+    var href = getBaseElementHref();
+    if (href == null) {
+      return null;
+    }
+    var baseUri = Uri.parse(href);
     return baseUri.path;
   }
   String getUserAgent() {
@@ -360,3 +363,15 @@ class BrowserDomAdapter extends GenericBrowserDomAdapter {
     js.context[name] = value;
   }
 }
+
+
+var baseElement = null;
+String getBaseElementHref() {
+  if (baseElement == null) {
+    baseElement = document.querySelector('base');
+    if (baseElement == null) {
+      return null;
+    }
+  }
+  return baseElement.getAttribute('href');
+}

modules/angular2/src/dom/browser_adapter.ts

@@ -263,14 +263,32 @@ export class BrowserDomAdapter extends GenericBrowserDomAdapter {
   }
   getHistory(): History { return window.history; }
   getLocation(): Location { return window.location; }
-  getBaseHref(): string { return relativePath(document.baseURI); }
+  getBaseHref(): string {
+    var href = getBaseElementHref();
+    if (isBlank(href)) {
+      return null;
+    }
+    return relativePath(href);
+  }
   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]; }
   // TODO(tbosch): move this into a separate environment class once we have it
   setGlobalVar(name: string, value: any) { global[name] = value; }
 }
 
+
+var baseElement = null;
+function getBaseElementHref(): string {
+  if (isBlank(baseElement)) {
+    baseElement = document.querySelector('base');
+    if (isBlank(baseElement)) {
+      return null;
+    }
+  }
+  return baseElement.attr('href');
+}
+
 // based on urlUtils.js in AngularJS 1
 var urlParsingNode = null;
 function relativePath(url): string {