Make StringSource fully implement Source (issue 25822)

pkg/analyzer/lib/src/string_source.dart

 // Copyright (c) 2013, 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.string_source;
 
 import 'package:analyzer/src/generated/engine.dart' show TimestampedData;
 import 'package:analyzer/src/generated/source.dart';
 
 /**
  * An implementation of [Source] that's based on an in-memory Dart string.
  */
 class StringSource extends Source {
   /**
    * The content of the source.
    */
   final String _contents;
 
   @override
   final String fullName;
 
   @override
+  final Uri uri;
+
+  @override
   final int modificationStamp;
 
-  StringSource(this._contents, this.fullName)
-      : modificationStamp = new DateTime.now().millisecondsSinceEpoch;
+  StringSource(this._contents, String fullName)
+      : this.fullName = fullName,
+        uri = fullName == null ? null : new Uri.file(fullName),
+        modificationStamp = new DateTime.now().millisecondsSinceEpoch;
 
   @override
   TimestampedData<String> get contents =>
       new TimestampedData(modificationStamp, _contents);
 
   @override
-  String get encoding =>
-      throw new UnsupportedError("StringSource doesn't support encoding.");
+  String get encoding => uri.toString();
 
   @override
   int get hashCode => _contents.hashCode ^ fullName.hashCode;
 
   @override
   bool get isInSystemLibrary => false;
 
   @override
   String get shortName => fullName;
 
   @override
-  Uri get uri =>
-      throw new UnsupportedError("StringSource doesn't support uri.");
-
-  UriKind get uriKind =>
-      throw new UnsupportedError("StringSource doesn't support uriKind.");
+  UriKind get uriKind => UriKind.FILE_URI;
 
   /**
    * Return `true` if the given [object] is a string source that is equal to
    * this source.
    */
   bool operator ==(Object object) {
     return object is StringSource &&
         object._contents == _contents &&
         object.fullName == fullName;
   }
 
   @override
   bool exists() => true;
 
   @override
-  Uri resolveRelativeUri(Uri relativeUri) => throw new UnsupportedError(
-      "StringSource doesn't support resolveRelative.");
+  Uri resolveRelativeUri(Uri relativeUri) => uri.resolveUri(relativeUri);
 
   @override
   String toString() => 'StringSource ($fullName)';
 }