Remove two unused constructors named '_none' (#979)

Author: srawlins

lib/src/async_import_cache.dart

@@ -94,14 +94,6 @@ class AsyncImportCache {
     ];
   }
 
-  /// Creates a cache that contains no importers.
-  const AsyncImportCache._none()
-      : _importers = const [],
-        _logger = const Logger.stderr(),
-        _canonicalizeCache = const {},
-        _importCache = const {},
-        _resultsCache = const {};
-
   /// Canonicalizes [url] according to one of this cache's importers.
   ///
   /// Returns the importer that was used to canonicalize [url], the canonical

lib/src/import_cache.dart

@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_import_cache.dart.
 // See tool/grind/synchronize.dart for details.
 //
-// Checksum: c1dd0f2a41b03bd1227a2358688bf44499b9d90c
+// Checksum: c39f68f9964e4ae03c6447d845349e276def5031
 //
 // ignore_for_file: unused_import
 
@@ -98,14 +98,6 @@ class ImportCache {
     ];
   }
 
-  /// Creates a cache that contains no importers.
-  const ImportCache._none()
-      : _importers = const [],
-        _logger = const Logger.stderr(),
-        _canonicalizeCache = const {},
-        _importCache = const {},
-        _resultsCache = const {};
-
   /// Canonicalizes [url] according to one of this cache's importers.
   ///
   /// Returns the importer that was used to canonicalize [url], the canonical