Implement UriResolver.restoreAbsolute() for DartUriResolver.

pkg/analyzer/lib/src/generated/source.dart

Index: pkg/analyzer/lib/src/generated/source.dart
diff --git a/pkg/analyzer/lib/src/generated/source.dart b/pkg/analyzer/lib/src/generated/source.dart
index 83a4783942ddade0aeb3bf5aabe0c0e34d77c51f..262bfe3b2d79c0b3b4398035afc2a321d4757b1c 100644
--- a/pkg/analyzer/lib/src/generated/source.dart
+++ b/pkg/analyzer/lib/src/generated/source.dart
@@ -174,6 +174,12 @@ class DartUriResolver extends UriResolver {
     return _sdk.mapDartUri(uri.toString());
   }
 
+  @override
+  Uri restoreAbsolute(Source source) {
+    Source dartSource = _sdk.fromFileUri(source.uri);
+    return dartSource?.uri;
+  }
+
   /**
    * Return `true` if the given URI is a `dart-ext:` URI.
    *