use type information to filter RHS of "is" expression for imported types

pkg/analysis_server/lib/src/services/completion/dart/imported_reference_contributor.dart

Index: pkg/analysis_server/lib/src/services/completion/dart/imported_reference_contributor.dart
diff --git a/pkg/analysis_server/lib/src/services/completion/dart/imported_reference_contributor.dart b/pkg/analysis_server/lib/src/services/completion/dart/imported_reference_contributor.dart
index 6a8c367aa39e07ea48c39108f1eda4908a231305..46514910e3633f3e91134c24bb54d7482e24554b 100644
--- a/pkg/analysis_server/lib/src/services/completion/dart/imported_reference_contributor.dart
+++ b/pkg/analysis_server/lib/src/services/completion/dart/imported_reference_contributor.dart
@@ -10,6 +10,7 @@ import 'package:analysis_server/src/provisional/completion/dart/completion_dart.
 import 'package:analysis_server/src/services/completion/dart/completion_manager.dart';
 import 'package:analysis_server/src/services/completion/dart/local_library_contributor.dart';
 import 'package:analysis_server/src/services/completion/dart/optype.dart';
+import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/generated/resolver.dart';
 
@@ -53,6 +54,20 @@ class ImportedReferenceContributor extends DartCompletionContributor {
       return EMPTY_LIST;
     }
 
+    // If the target is in an expression
+    // then resolve the outermost/entire expression
+    AstNode node = request.target.containingNode;
+    if (node is Expression) {
+      while (node.parent is Expression) {
+        node = node.parent;
+      }
+      await request.resolveExpression(node);
+
+      // Discard any cached target information
+      // because it may have changed as a result of the resolution
+      node = request.target.containingNode;
+    }
+
     this.request = request;
     this.optype = (request as DartCompletionRequestImpl).opType;
     List<CompletionSuggestion> suggestions = <CompletionSuggestion>[];