Improve handling of locations not from the uris the source map is for.

test/parser_test.dart

Index: test/parser_test.dart
diff --git a/test/parser_test.dart b/test/parser_test.dart
index 9448ea08fa19ff0a51967b2b8465ced3d41e2bb2..a317f7e6370382578ba75a645a78f34b972c83df 100644
--- a/test/parser_test.dart
+++ b/test/parser_test.dart
@@ -178,7 +178,15 @@ main() {
     });
 
     test('unmapped path', () {
-      expect(mapping.spanFor(0, 0, uri: "unmapped_output.dart"), isNull);
+      var span = mapping.spanFor(0, 0, uri: "unmapped_output.dart");
+      expect(span.sourceUrl, Uri.parse("unmapped_output.dart"));
+      expect(span.start.line, equals(0));
+      expect(span.start.column, equals(0));
+
+      span = mapping.spanFor(10, 5, uri: "unmapped_output.dart");
+      expect(span.sourceUrl, Uri.parse("unmapped_output.dart"));
+      expect(span.start.line, equals(10));
+      expect(span.start.column, equals(5));
     });
 
     test('missing path', () {