Teach SkyView code path to print hello, world

sky/engine/core/script/dom_dart_state.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "sky/engine/config.h"
 #include "sky/engine/core/script/dom_dart_state.h"
 
 #include "sky/engine/core/dom/Document.h"
 #include "sky/engine/core/script/dart_loader.h"
 
 namespace blink {
 
 DOMDartState::DOMDartState(Document* document)
     : document_(document), loader_(adoptPtr(new DartLoader(this))) {
-  DCHECK(document_);
 }
 
 DOMDartState::~DOMDartState() {
 }
 
 DOMDartState* DOMDartState::Current() {
   return static_cast<DOMDartState*>(DartState::Current());
 }
 
 Document* DOMDartState::CurrentDocument() {
   return Current()->document_.get();
 }
 
 LocalFrame* DOMDartState::CurrentFrame() {
+  DCHECK(Current()->document_);
   return Current()->document_->frame();
 }
 
 LocalDOMWindow* DOMDartState::CurrentWindow() {
+  DCHECK(Current()->document_);
   return Current()->document_->domWindow();
 }
 
 }