Allow for no embedder provided library tag handler

Dartium doesn't have a library tag handler installed in the service isolate. Allow for no embedder provided library tag handler so long as no imports need to be handled.

Fixes part of https://code.google.com/p/dart/issues/detail?id=20049

R=asiva@google.com

Committed: https://code.google.com/p/dart/source/detail?r=38239

[siva, 2014-07-15 16:06:54]

LGTM with one comment.

https://codereview.chromium.org/391943003/diff/1/runtime/vm/service.cc
File runtime/vm/service.cc (right):

https://codereview.chromium.org/391943003/diff/1/runtime/vm/service.cc#newcod...
runtime/vm/service.cc:572: ASSERT(embedder_provided_handler_ != NULL);
instead of asserting here why not return an error here if
embedder_provided_handler_ is NULL

something like:

return Dart_NewApiError("Unable to import module %s as no library tag handler
has been registered", utl_string);

[Cutch, 2014-07-15 16:33:51]

https://codereview.chromium.org/391943003/diff/1/runtime/vm/service.cc
File runtime/vm/service.cc (right):

https://codereview.chromium.org/391943003/diff/1/runtime/vm/service.cc#newcod...
runtime/vm/service.cc:572: ASSERT(embedder_provided_handler_ != NULL);
On 2014/07/15 16:06:54, siva wrote:
> instead of asserting here why not return an error here if
> embedder_provided_handler_ is NULL
> 
> something like:
> 
> return Dart_NewApiError("Unable to import module %s as no library tag handler
> has been registered", utl_string);

Done.

[Cutch, 2014-07-15 16:34:01]

Committed patchset #2 manually as r38239 (presubmit successful).