Remove a couple of unnecessary is checks

pkg/analyzer/lib/src/context/context.dart

Index: pkg/analyzer/lib/src/context/context.dart
diff --git a/pkg/analyzer/lib/src/context/context.dart b/pkg/analyzer/lib/src/context/context.dart
index 51bcc57d787862e5122030eef00ccc8c492f8d82..e5224838b8c4a753428d574cde103a260242ee2e 100644
--- a/pkg/analyzer/lib/src/context/context.dart
+++ b/pkg/analyzer/lib/src/context/context.dart
@@ -809,13 +809,14 @@ class AnalysisContextImpl implements InternalAnalysisContext {
     CacheEntry entry = _cache.get(target);
     if (entry == null) {
       entry = new CacheEntry(target);
+      ImplicitAnalysisEvent event = null;
       if (target is Source) {
         entry.modificationTime = getModificationStamp(target);
+        event = new ImplicitAnalysisEvent(target, true);
       }
       _cache.put(entry);
-      if (target is Source) {
-        _implicitAnalysisEventsController
-            .add(new ImplicitAnalysisEvent(target, true));
+      if (event != null) {
+        _implicitAnalysisEventsController.add(event);
       }
     }
     return entry;