Fix the resolution of uri-based part-of directives in the SDK (issue 29598)

pkg/analyzer/lib/src/dart/sdk/sdk.dart

 // Copyright (c) 2016, 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.generated.sdk2;
 
 import 'dart:collection';
 import 'dart:convert';
 import 'dart:io' as io;
 
@@ skipping 633 matching lines @@
       libraryName = dartUri;
       relativePath = "";
     }
     SdkLibrary library = getSdkLibrary(libraryName);
     if (library == null) {
       return null;
     }
     try {
       File file = libraryDirectory.getChildAssumingFile(library.path);
       if (!relativePath.isEmpty) {
-        file = file.parent.getChildAssumingFile(relativePath);
+        File relativeFile = file.parent.getChildAssumingFile(relativePath);
+        if (relativeFile.path == file.path) {
+          // The relative file is the library, so return a Source for the
+          // library rather than the part format.
+          return file.createSource(Uri.parse(library.shortName));
+        }
+        file = relativeFile;
       }
       return file.createSource(Uri.parse(dartUri));
     } on FormatException {
       return null;
     }
   }
 
   /**
    * Return the default directory for the Dart SDK, or `null` if the directory
    * cannot be determined (or does not exist). The default directory is provided
@@ skipping 227 matching lines @@
     SdkLibrariesReader_LibraryBuilder libraryBuilder =
         new SdkLibrariesReader_LibraryBuilder(_useDart2jsPaths);
     // If any syntactic errors were found then don't try to visit the AST
     // structure.
     if (!errorListener.errorReported) {
       unit.accept(libraryBuilder);
     }
     return libraryBuilder.librariesMap;
   }
 }