Completion breaks if core is not analyzed (issue 22814)

pkg/analysis_server/lib/src/services/completion/dart_completion_cache.dart

Index: pkg/analysis_server/lib/src/services/completion/dart_completion_cache.dart
diff --git a/pkg/analysis_server/lib/src/services/completion/dart_completion_cache.dart b/pkg/analysis_server/lib/src/services/completion/dart_completion_cache.dart
index ca9c75af65e6c471ad515c08355faa971a30fec0..9cc5f663d05dbc5a8e0636aa3ea9ac2eb5e78327 100644
--- a/pkg/analysis_server/lib/src/services/completion/dart_completion_cache.dart
+++ b/pkg/analysis_server/lib/src/services/completion/dart_completion_cache.dart
@@ -198,6 +198,11 @@ class DartCompletionCache extends CompletionCache {
   void _addDartCoreSuggestions() {
     Source coreUri = context.sourceFactory.forUri('dart:core');
     LibraryElement coreLib = context.getLibraryElement(coreUri);
+    if (coreLib == null) {
+      // If the core library has not been analyzed yet, then we cannot add any
+      // suggestions from it.
+      return;
+    }
     Namespace coreNamespace =
         new NamespaceBuilder().createPublicNamespaceForLibrary(coreLib);
     coreNamespace.definedNames.forEach((String name, Element elem) {