fix(test_lib): run unit tests in default Documnent

Closes #3501
Fixes #3475

modules/angular2/src/test_lib/test_component_builder.ts

@@ -138,6 +138,10 @@ export class TestComponentBuilder {
     var doc = this._injector.get(DOCUMENT_TOKEN);
 
     // TODO(juliemr): can/should this be optional?
+    var oldRoots = DOM.querySelectorAll(doc, '[id^=root]');
+    for (var i = 0; i < oldRoots.length; i++) {
+      DOM.remove(oldRoots[i]);
+    }
     DOM.appendChild(doc.body, rootEl);
 
 

modules/angular2/src/test_lib/test_injector.ts

@@ -93,7 +93,7 @@ function _getAppBindings() {
 
   // The document is only available in browser environment
   try {
-    appDoc = DOM.createHtmlDocument();
+    appDoc = DOM.defaultDoc();
   } catch (e) {
     appDoc = null;
   }