Update uses of deprecated API in ddc

pkg/analyzer/lib/source/custom_resolver.dart

+// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library analyzer.source.custom_resolver;
+
+import 'package:analyzer/file_system/file_system.dart';
+import 'package:analyzer/src/generated/source.dart';
+
+/**
+ * A resolver that maps individual URI's to other URI's.

[scheglov, 2016/09/13 18:27:24]

The comment seems too generic.

[Brian Wilkerson, 2016/09/13 20:37:20]

The original class had no comment, and I don't actually understand the purpose
of the class, so I've removed the comment.

+ */
+class CustomUriResolver extends UriResolver {
+  final ResourceProvider resourceProvider;
+  final Map<String, String> _urlMappings;
+
+  CustomUriResolver(this.resourceProvider, this._urlMappings);
+
+  @override
+  Source resolveAbsolute(Uri uri, [Uri actualUri]) {
+    String mapping = _urlMappings[uri.toString()];
+    if (mapping == null) {
+      return null;
+    }
+    Uri fileUri = new Uri.file(mapping);
+    if (!fileUri.isAbsolute) {
+      return null;
+    }
+    return resourceProvider
+        .getFile(fileUri.toFilePath())

[scheglov, 2016/09/13 18:27:24]

I think we need to use ResourceProvider's path context.

[Brian Wilkerson, 2016/09/13 20:37:20]

Done

+        .createSource(actualUri ?? uri);
+  }
+}