Update the plugin API

pkg/analyzer_plugin/lib/utilities/completion/completion_core.dart

Index: pkg/analyzer_plugin/lib/utilities/completion/completion_core.dart
diff --git a/pkg/analyzer_plugin/lib/utilities/completion/completion_core.dart b/pkg/analyzer_plugin/lib/utilities/completion/completion_core.dart
index 6e90bb6a51d956400f42d22f747f162273f2c2d4..bb2b3c8af2a263adabbfa45c698d307595453c4e 100644
--- a/pkg/analyzer_plugin/lib/utilities/completion/completion_core.dart
+++ b/pkg/analyzer_plugin/lib/utilities/completion/completion_core.dart
@@ -73,7 +73,7 @@ abstract class CompletionContributor {
    * the given [request] into the given [collector].
    */
   Future<Null> computeSuggestions(
-      CompletionRequest request, CompletionCollector collector);
+      covariant CompletionRequest request, CompletionCollector collector);
 }
 
 /**
@@ -142,13 +142,21 @@ abstract class CompletionRequest {
   ResourceProvider get resourceProvider;
 
   /**
-   * The analysis result for the file in which the completion is being
-   * requested.
+   * Throw an [AbortCompletion] if the completion request has been aborted.
    */
-  ResolveResult get result;
+  void checkAborted();
+}
 
+/**
+ * The information about a requested list of completions when completing in a
+ * `.dart` file.
+ *
+ * Clients may not extend, implement or mix-in this class.
+ */
+abstract class DartCompletionRequest implements CompletionRequest {
   /**
-   * Throw an [AbortCompletion] if the completion request has been aborted.
+   * The analysis result for the file in which the completion is being
+   * requested.
    */
-  void checkAborted();
+  ResolveResult get result;
 }