rework LocalLibraryContributor to resolve referenced units

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 c6a9a7489ba9a93dce50edd61535dfbb3febc657..2d42cbbea6111168429aa7b8310aa7ef32fe23fa 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
@@ -125,4 +125,14 @@ abstract class DartCompletionRequest extends CompletionRequest {
    * should be discarded as it may have changed.
    */
   Future resolveExpression(Expression expression);
+
+  /**
+   * Return a [Future] that completes with a list of [CompilationUnitElement]s
+   * comprising the library in which in which the completion is occurring.
+   * The [Future] may return `null` if the library unit cannot be determined
+   * (e.g. unlinked part file).
+   * Any information obtained from [target] prior to calling this method
+   * should be discarded as it may have changed.
+   */
+  Future<List<CompilationUnitElement>> resolveUnits();
 }