fix call to get current source - fixes #24509

R=scheglov@google.com

Review URL: https://codereview.chromium.org/1385213002 .

Author: Dan Rubel

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

@@ -356,7 +356,7 @@ class _LocalVisitor extends LocalDeclarationVisitor {
   _LocalVisitor(this.request, int offset, this.optype) : super(offset) {
     includeLocalInheritedTypes = !optype.inStaticMethodBody;
     if (request.replacementLength > 0) {
-      var contents = request.source.contents;
+      var contents = request.context.getContents(request.source);
       if (contents != null &&
           contents.data != null &&
           contents.data.startsWith('_', request.replacementOffset)) {