Improve DartLoader error handling.

mojo/common/data_pipe_drainer.cc

 // Copyright 2014 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 "mojo/common/data_pipe_drainer.h"
 #include "base/bind.h"
 
 namespace mojo {
 namespace common {
 
@@ skipping 16 matching lines @@
       &buffer, &num_bytes, MOJO_READ_DATA_FLAG_NONE);
   if (rv == MOJO_RESULT_OK) {
     client_->OnDataAvailable(buffer, num_bytes);
     EndReadDataRaw(source_.get(), num_bytes);
     WaitForData();
   } else if (rv == MOJO_RESULT_SHOULD_WAIT) {
     WaitForData();
   } else if (rv == MOJO_RESULT_FAILED_PRECONDITION) {
     client_->OnDataComplete();
   } else {
-    DCHECK(false);
+    DCHECK(false) << "Unhandled MojoResult: " << rv;
   }
 }
 
 void DataPipeDrainer::WaitForData() {
   handle_watcher_.Start(source_.get(),
       MOJO_HANDLE_SIGNAL_READABLE, MOJO_DEADLINE_INDEFINITE,
       base::Bind(&DataPipeDrainer::WaitComplete, weak_factory_.GetWeakPtr()));
 }
 
 void DataPipeDrainer::WaitComplete(MojoResult result) {
   ReadData();
 }
 
 }  // namespace common
 }  // namespace mojo