Handle context == null in the analysis.getHover request.

pkg/analysis_server/lib/src/domain_analysis.dart

Index: pkg/analysis_server/lib/src/domain_analysis.dart
diff --git a/pkg/analysis_server/lib/src/domain_analysis.dart b/pkg/analysis_server/lib/src/domain_analysis.dart
index db34aa7a04de9562489632947b48e7268a68ff40..5b7cb443ea07314d7f18d69dba5a8bb559b0dd1d 100644
--- a/pkg/analysis_server/lib/src/domain_analysis.dart
+++ b/pkg/analysis_server/lib/src/domain_analysis.dart
@@ -41,11 +41,13 @@ class AnalysisDomainHandler implements RequestHandler {
     {
       Source source = server.getSource(file);
       AnalysisContext context = server.getAnalysisContext(file);
-      List<Source> librarySources = context.getLibrariesContaining(source);
-      for (Source librarySource in librarySources) {
-        CompilationUnit unit = context.resolveCompilationUnit2(source,
-            librarySource);
-        hovers.add(new DartUnitHoverComputer(unit, offset).compute());
+      if (context != null) {
+        List<Source> librarySources = context.getLibrariesContaining(source);
+        for (Source librarySource in librarySources) {
+          CompilationUnit unit = context.resolveCompilationUnit2(source,
+              librarySource);
+          hovers.add(new DartUnitHoverComputer(unit, offset).compute());
+        }
       }
     }
     // send response