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

test/parser_test.dart

 // Copyright (c) 2013, 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 test.parser_test;
 
 import 'dart:convert';
 import 'package:test/test.dart';
 import 'package:source_maps/source_maps.dart';
 import 'package:source_span/source_span.dart';
@@ skipping 160 matching lines @@
 
       expect(
           mapping.spanFor(0, 0, uri: "file:///path/to/output1.dart").sourceUrl,
           Uri.parse("file:///path/to/pkg/input1.dart"));
       expect(
           mapping.spanFor(0, 0, uri: "file:///path/to/output2.dart").sourceUrl,
           Uri.parse("file:///path/to/pkg/input2.dart"));
     });
 
     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', () {
       expect(() => mapping.spanFor(0, 0), throws);
     });
 
     test('incomplete paths', () {
       expect(mapping.spanFor(0, 0, uri: "output1.dart").sourceUrl,
           Uri.parse("file:///path/to/pkg/input1.dart"));
       expect(mapping.spanFor(0, 0, uri: "output2.dart").sourceUrl,
@@ skipping 49 matching lines @@
       mapping = parseJsonExtended(expected);
       expect(mapping.toJson(), equals(expected));
     }
     // Invalid for this case
     expect(() => parseJson(SOURCE_MAP_BUNDLE), throws);
 
     var mapping = parseJsonExtended(SOURCE_MAP_BUNDLE);
     expect(mapping.toJson(), equals(SOURCE_MAP_BUNDLE));
   });
 }