Address review comments

Author: ianjoneill

src/lib/sidenav/sidenav.html

@@ -1,4 +1,4 @@
-<div class="md-sidenav-backdrop" (click)="_closeModalSidenav()"
+<div class="md-sidenav-backdrop" (click)="_onBackdropClicked()"
      [class.md-sidenav-shown]="_isShowingBackdrop()"></div>
 
 <ng-content select="md-sidenav"></ng-content>

src/lib/sidenav/sidenav.ts

@@ -400,9 +400,12 @@ export class MdSidenavLayout implements AfterContentInit {
     this._setDrawersValid(true);
   }
 
-  _closeModalSidenav() {
-    this.onBackdropClicked.emit(null);
+  _onBackdropClicked() {
+    this.onBackdropClicked.emit();
+    this._closeModalSidenav();
+  }
 
+  _closeModalSidenav() {
     if (this._start != null && this._start.mode != 'side') {
       this._start.close();
     }