fix embedder on Windows - fixes #25498

pkg/analyzer/lib/source/embedder.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.source.embedder;
 
 import 'dart:collection' show HashMap;
 import 'dart:core' hide Resource;
 
 import 'package:analyzer/file_system/file_system.dart';
@@ skipping 138 matching lines @@
     SdkLibraryImpl library = new SdkLibraryImpl(shortName);
     library.path = libPath;
     (dartSdk as EmbedderSdk)._librariesMap.setLibrary(name, library);
   }
 
   /// Number of embedder libraries.
   int get length => _urlMappings.length;
 
   @override
   Uri restoreAbsolute(Source source) {
-    Source sdkSource = dartSdk.fromFileUri(Uri.parse(source.fullName));
+    String path = source.fullName;
+    if (path.length > 3 && path[1] == ':' && path[2] == '\\') {
+      path = '/${path[0]}:${path.substring(2).replaceAll('\\', '/')}';
+    }
+    Source sdkSource = dartSdk.fromFileUri(Uri.parse('file://$path'));
     return sdkSource?.uri;
   }
 }
 
 class EmbedderSdk implements DartSdk {
   // TODO(danrubel) Refactor this with DirectoryBasedDartSdk
 
   /// The resolver associated with this embedder sdk.
   EmbedderUriResolver _resolver;
 
@@ skipping 110 matching lines @@
     }
     String filePath = srcPath.replaceAll('/', JavaFile.separator);
     try {
       JavaFile file = new JavaFile(filePath);
       return new FileBasedSource(file, parseUriWithException(dartUri));
     } on URISyntaxException {
       return null;
     }
   }
 }