Fix for 'Create part' Quick Fix.

pkg/analyzer/lib/source/package_map_resolver.dart

 // Copyright (c) 2014, 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 source.package_map_resolver;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/util/asserts.dart' as asserts;
 import 'package:path/path.dart' as pathos;
@@ skipping 53 matching lines @@
           Resource result = packageDir.getChild(relPath);
           if (result is File && result.exists) {
             return result.createSource(uri);
           }
         }
       }
     }
     // Return a NonExistingSource instance.
     // This helps provide more meaningful error messages to users
     // (a missing file error, as opposed to an invalid URI error).
-    return new NonExistingSource(uri.toString(), UriKind.PACKAGE_URI);
+    String fullPath = packageDirs != null && packageDirs.isNotEmpty
+        ? packageDirs.first.canonicalizePath(relPath)
+        : relPath;
+    return new NonExistingSource(fullPath, uri, UriKind.PACKAGE_URI);
   }
 
   @override
   Uri restoreAbsolute(Source source) {
     String sourcePath = source.fullName;
     Uri bestMatch;
     int bestMatchLength = -1;
     pathos.Context pathContext = resourceProvider.pathContext;
     for (String pkgName in packageMap.keys) {
       List<Folder> pkgFolders = packageMap[pkgName];
@@ skipping 38 matching lines @@
     return true;
   }
 
   /**
    * Returns `true` if [uri] is a `package` URI.
    */
   static bool isPackageUri(Uri uri) {
     return uri.scheme == PACKAGE_SCHEME;
   }
 }