Better error message when attempting to import dart:html on the stand-alone VM.

runtime/bin/dartutils.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "bin/dartutils.h"
 
 #include "include/dart_api.h"
 #include "include/dart_native_api.h"
 
 #include "platform/assert.h"
@@ skipping 467 matching lines @@
     return ResolveUri(library_url, url, builtin_lib);
   }
 
   // Handle 'import' of dart scheme URIs (i.e they start with 'dart:').
   if (is_dart_scheme_url) {
     if (tag == Dart_kImportTag) {
       // Handle imports of other built-in libraries present in the SDK.
       if (DartUtils::IsDartIOLibURL(url_string)) {
         return Builtin::LoadAndCheckLibrary(Builtin::kIOLibrary);
       }
-      return Dart_Error("Do not know how to load '%s'", url_string);
+      return Dart_Error("The built-in library '%s' is not available"
+                        " on the stand-alone VM.\n", url_string);
     } else {
       ASSERT(tag == Dart_kSourceTag);
       return Dart_Error("Unable to load source '%s' ", url_string);
     }
   }
 
   // Handle 'part' of IO library.
   if (is_io_library) {
     if (tag == Dart_kSourceTag) {
       return Dart_LoadSource(
@@ skipping 524 matching lines @@
       new CObjectString(CObject::NewString(os_error->message()));
   CObjectArray* result = new CObjectArray(CObject::NewArray(3));
   result->SetAt(0, new CObjectInt32(CObject::NewInt32(kOSError)));
   result->SetAt(1, new CObjectInt32(CObject::NewInt32(os_error->code())));
   result->SetAt(2, error_message);
   return result;
 }
 
 }  // namespace bin
 }  // namespace dart