fix SourceFactoryImpl restoreUri

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

 // Copyright (c) 2015, 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.src.context.source;
 
 import 'dart:collection';
+import 'dart:math' show min;
 
 import 'package:analyzer/exception/exception.dart';
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/physical_file_system.dart';
 import 'package:analyzer/source/package_map_resolver.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/sdk.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/generated/utilities_dart.dart' as utils;
 import 'package:analyzer/src/util/fast_uri.dart';
@@ skipping 178 matching lines @@
     }
     if (sourceUri.scheme != 'file') {
       //TODO(pquitslund): verify this works for non-file URIs.
       return null;
     }
 
     Uri packageUri;
     _packages.asMap().forEach((String name, Uri uri) {
       if (packageUri == null) {
         if (utils.startsWith(sourceUri, uri)) {
-          packageUri = Uri.parse(
-              'package:$name/${sourceUri.path.substring(uri.path.length)}');
+          String relativePath = sourceUri.path
+              .substring(min(uri.path.length, sourceUri.path.length));
+          packageUri = Uri.parse('package:$name/$relativePath');
         }
       }
     });
     return packageUri;
   }
 
   /**
    * Return a source object representing the URI that results from resolving
    * the given (possibly relative) contained URI against the URI associated
    * with an existing source object, or `null` if the URI could not be resolved.
@@ skipping 41 matching lines @@
       for (UriResolver resolver in resolvers) {
         Source result = resolver.resolveAbsolute(containedUri, actualUri);
         if (result != null) {
           return result;
         }
       }
       return null;
     });
   }
 }