Fix to closure mock.dart

pkg/kernel/lib/transformations/closure/mock.dart

 // Copyright (c) 2016, 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 kernel.transformations.closure.mock;
 
 import '../../ast.dart'
     show
         Arguments,
         Block,
@@ skipping 11 matching lines @@
         Initializer,
         IntLiteral,
         Library,
         MethodInvocation,
         Name,
         Procedure,
         ProcedureKind,
         Program,
         PropertyGet,
         ReturnStatement,
+        Source,
         Statement,
         StaticInvocation,
         Supertype,
         VariableDeclaration,
         VariableGet;
 
 import '../../core_types.dart' show CoreTypes;
 
 import '../../frontend/accessors.dart'
     show
@@ skipping 134 matching lines @@
       name: "Context",
       supertype: new Supertype(coreTypes.objectClass, const <DartType>[]),
       constructors: [constructor],
       fields: fields,
       procedures: procedures,
       fileUri: fileUri);
   Library mock = new Library(Uri.parse(fileUri),
       name: "mock", classes: [contextClass])..fileUri = fileUri;
   program.libraries.add(mock);
   mock.parent = program;
-  program.uriToLineStarts[mock.fileUri] = <int>[0];
+  program.uriToSource[mock.fileUri] = new Source(<int>[0], "");
   return contextClass;
 }