Move SdkExtUriResolver from analysis_server to analyzer so it can be used in analyzer_cli

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 analysis_server.src.sdk_ext;
+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;
@@ skipping 142 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;
   }
 }