Fix an npe when Declaration._relevanceTags is null

There are several internal and external issues reported because of this
bug.

Change-Id: Ic166c0cf4ba15148e820878c0305249b4e26c2af
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/154261
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>

Author: bwilkerson

pkg/analyzer/lib/src/services/available_declarations.dart

@@ -57,7 +57,7 @@ class Declaration {
   final String returnType;
   final String typeParameters;
 
-  final List<String> _relevanceTags;
+  List<String> _relevanceTags;
   Uri _locationLibraryUri;
 
   Declaration({
@@ -1687,6 +1687,7 @@ class _File {
     for (var declaration in declarations) {
       var tags = RelevanceTags._forDeclaration(uriStr, declaration);
       if (tags != null) {
+        declaration._relevanceTags ??= [];
         declaration._relevanceTags.addAll(tags);
       }
       _computeRelevanceTags(declaration.children);