Refactor SourceFile, SourceFileProvider and SourceLocation.

tests/compiler/dart2js/memory_source_file_helper.dart

Index: tests/compiler/dart2js/memory_source_file_helper.dart
diff --git a/tests/compiler/dart2js/memory_source_file_helper.dart b/tests/compiler/dart2js/memory_source_file_helper.dart
index 827e5c1563c5b220dfd68ac4bdf5182f8ded9c5d..4efb8ad4466abe0c4d9e1746d5d2ce00c2e1da7c 100644
--- a/tests/compiler/dart2js/memory_source_file_helper.dart
+++ b/tests/compiler/dart2js/memory_source_file_helper.dart
@@ -35,8 +35,8 @@ class MemorySourceFileProvider extends SourceFileProvider {
     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);
   }