Second attempt at fixing SdkExtUriResolver.restoreAbsolute

pkg/analyzer/lib/source/sdk_ext.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 source.sdk_ext;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:convert';
 import 'dart:core' hide Resource;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/source/package_map_resolver.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/generated/java_io.dart' show JavaFile;
 import 'package:analyzer/src/generated/source_io.dart' show FileBasedSource;
 import 'package:path/path.dart' as pathos;
 
-class _SdkExtFileBasedSource extends FileBasedSource {
-  _SdkExtFileBasedSource(JavaFile file, Uri uri)
-      : super(file, uri);
-}
-
 /// Given a packageMap (see [PackageMapProvider]), check in each package's lib
 /// directory for the existence of a `.sdkext` file. This file must contain a
 /// JSON encoded map. Each key in the map is a `dart:` library name. Each value
 /// is a path (relative to the directory containing `.sdkext`) to a dart script
 /// for the given library. For example:
 /// {
 ///   "dart:sky": "../sdk_ext/dart_sky.dart"
 /// }
 ///
 /// If a key doesn't begin with `dart:` it is ignored.
@@ skipping 24 matching lines @@
   /// Number of sdk extensions.
   int get length => _urlMappings.length;
 
   /// Resolve a 'part' statement inside an sdk extension.
   Source _resolvePart(Uri libraryEntry, String partPath, Uri importUri) {
     // Library part.
     var directory = pathos.dirname(libraryEntry.path);
     var partUri = new Uri.file(pathos.join(directory, partPath));
     assert(partUri.isAbsolute);
     JavaFile javaFile = new JavaFile.fromUri(partUri);
-    return new _SdkExtFileBasedSource(javaFile, importUri);
+    return new FileBasedSource(javaFile, importUri);
   }
 
   /// Resolve an import of an sdk extension.
   Source _resolveEntry(Uri libraryEntry, Uri importUri) {
     // Library entry.
     JavaFile javaFile = new JavaFile.fromUri(libraryEntry);
-    return new _SdkExtFileBasedSource(javaFile, importUri);
+    return new FileBasedSource(javaFile, importUri);
+  }
+
+  /// Return the library name of [importUri].
+  String _libraryName(Uri importUri) {
+    var uri = importUri.toString();
+    int index = uri.indexOf('/');
+    if (index >= 0) {
+      return uri.substring(0, index);
+    }
+    return uri;
+  }
+
+  /// Return the part path of [importUri].
+  String _partPath(Uri importUri) {
+    var uri = importUri.toString();
+    int index = uri.indexOf('/');
+    if (index >= 0) {
+      return uri.substring(index + 1);
+    }
+    return null;
+  }
+
+  /// Returns true if [libraryName] is a registered sdk extension.
+  bool _registeredSdkExtension(String libraryName) {
+    return _urlMappings[libraryName] != null;
   }
 
   @override
   Source resolveAbsolute(Uri importUri) {
-    // Split import uri into library name and (optionally) a part path.
-    var uri = importUri.toString();
-    String libraryName;
-    String partPath;
-    int index = uri.indexOf('/');
-    if (index >= 0) {
-      libraryName = uri.substring(0, index);
-      partPath = uri.substring(index + 1);
-    } else {
-      libraryName = uri;
-    }
+    String libraryName = _libraryName(importUri);
+    String partPath = _partPath(importUri);
     // Lookup library name in mappings.
     String mapping = _urlMappings[libraryName];
     if (mapping == null) {
       // Not found.
       return null;
     }
     // This mapping points to the main entry file of the sdk extension.
     Uri libraryEntry = new Uri.file(mapping);
     if (!libraryEntry.isAbsolute) {
       // We expect an absolute path.
       return null;
     }
 
-    if (index >= 0) {
+    if (partPath != null) {
       return _resolvePart(libraryEntry, partPath, importUri);
     } else {
       return _resolveEntry(libraryEntry, importUri);
     }
   }
 
   @override
   Uri restoreAbsolute(Source source) {
-    if (source is _SdkExtFileBasedSource) {
+    String libraryName = _libraryName(source.uri);
+    if (_registeredSdkExtension(libraryName)) {
       return source.uri;
     }
     return null;
   }
 
   /// Given a package [name] and a list of folders ([libDirs]),
   /// add any found sdk extensions.
   void _processPackage(String name, List<Folder> libDirs) {
     for (var libDir in libDirs) {
       var sdkExt = _readDotSdkExt(libDir);
@@ skipping 34 matching lines @@
   void _processSdkExtension(String name, String file, Folder libDir) {
     if (!name.startsWith(DART_COLON_PREFIX)) {
       // SDK extensions must begin with 'dart:'.
       return;
     }
     var key = name;
     var value = libDir.canonicalizePath(file);
     _urlMappings[key] = value;
   }
 }