update completion cache key to include part directives

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 1eea8e87a8221d99f92e4645ca8ac2c3ad62fe08..7265c9d7bca98c95ebaf065b86f3fe839d74a72f 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
@@ -285,9 +285,7 @@ class DartCompletionCache extends CompletionCache {
   String _computeImportKey(CompilationUnit unit) {
     StringBuffer sb = new StringBuffer();
     unit.directives.forEach((Directive directive) {
-      if (directive is ImportDirective) {
-        sb.write(directive.toSource());
-      }
+      sb.write(directive.toSource());
     });
     return sb.toString();
   }