Don't compute 'dart:core' LibraryElement during completion.

pkg/analysis_server/lib/src/provisional/completion/dart/completion_dart.dart

Index: pkg/analysis_server/lib/src/provisional/completion/dart/completion_dart.dart
diff --git a/pkg/analysis_server/lib/src/provisional/completion/dart/completion_dart.dart b/pkg/analysis_server/lib/src/provisional/completion/dart/completion_dart.dart
index ade2e9f8ace1de1de3dc8c33b1fa89fc4e2b763f..093c2da1c662971059432075a1f2cac2c8993199 100644
--- a/pkg/analysis_server/lib/src/provisional/completion/dart/completion_dart.dart
+++ b/pkg/analysis_server/lib/src/provisional/completion/dart/completion_dart.dart
@@ -46,11 +46,6 @@ abstract class DartCompletionContributor {
  */
 abstract class DartCompletionRequest extends CompletionRequest {
   /**
-   * Return the dart:core library element
-   */
-  LibraryElement get coreLib;
-
-  /**
    * Return the expression to the right of the "dot" or "dot dot",
    * or `null` if this is not a "dot" completion (e.g. `foo.b`).
    */