Fix for subscribing for notifications for files in packages.

pkg/analysis_server/lib/src/domain_execution.dart

Index: pkg/analysis_server/lib/src/domain_execution.dart
diff --git a/pkg/analysis_server/lib/src/domain_execution.dart b/pkg/analysis_server/lib/src/domain_execution.dart
index d3323c70a15467550085d3505c1b407bba2bb508..08ff54e41f22a592e918d4d53510d70683519f7e 100644
--- a/pkg/analysis_server/lib/src/domain_execution.dart
+++ b/pkg/analysis_server/lib/src/domain_execution.dart
@@ -114,7 +114,8 @@ class ExecutionDomainHandler implements RequestHandler {
         return new Response.invalidParameter(
             request, 'file', 'Must not refer to a directory');
       }
-      Source source = server.getSource(file);
+      ContextSourcePair contextSource = server.getContextSourcePair(file);
+      Source source = contextSource.source;
       uri = context.sourceFactory.restoreUri(source).toString();
       return new ExecutionMapUriResult(uri: uri).toResponse(request.id);
     } else if (uri != null) {
@@ -157,6 +158,9 @@ class ExecutionDomainHandler implements RequestHandler {
         return;
       }
       AnalysisContext context = server.getAnalysisContext(filePath);
+      if (context == null) {
+        return;
+      }
       if (AnalysisEngine.isDartFileName(filePath)) {
         ExecutableKind kind = ExecutableKind.NOT_EXECUTABLE;
         if (context.isClientLibrary(source)) {