Refactor SourceFile, SourceFileProvider and SourceLocation.

tests/compiler/dart2js/memory_source_file_helper.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 dart2js.test.memory_source_file_helper;
 
 import 'dart:async' show Future;
 export 'dart:io' show Platform;
 
 export 'package:compiler/src/apiimpl.dart'
@@ skipping 17 matching lines @@
   MemorySourceFileProvider(Map<String, String> this.memorySourceFiles);
 
   Future<String> readStringFromUri(Uri resourceUri) {
     if (resourceUri.scheme != 'memory') {
       return super.readStringFromUri(resourceUri);
     }
     String source = memorySourceFiles[resourceUri.path];
     if (source == null) {
       return new Future.error(new Exception('No such file $resourceUri'));
     }
-    String resourceName = '$resourceUri';
-    this.sourceFiles[resourceName] = new StringSourceFile(resourceName, source);
+    this.sourceFiles[resourceUri] =
+        new StringSourceFile.fromUri(resourceUri, source);
     return new Future.value(source);
   }
 
   Future<String> call(Uri resourceUri) => readStringFromUri(resourceUri);
 }