Remove unnecessary casts and general code clean-up

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 e5224838b8c4a753428d574cde103a260242ee2e..7815864b88b325362543b8c4c5c2c1609dd299c3 100644
--- a/pkg/analyzer/lib/src/context/context.dart
+++ b/pkg/analyzer/lib/src/context/context.dart
@@ -374,8 +374,7 @@ class AnalysisContextImpl implements InternalAnalysisContext {
     List<Source> sources = <Source>[];
     for (Source source in _cache.sources) {
       CacheEntry entry = _cache.get(source);
-      if (source is Source &&
-          entry.getValue(SOURCE_KIND) == SourceKind.LIBRARY &&
+      if (entry.getValue(SOURCE_KIND) == SourceKind.LIBRARY &&
           !source.isInSystemLibrary &&
           isServerLibrary(source)) {
         sources.add(source);