Try again to fix the bots (TBR)

pkg/analyzer_plugin/test/support/abstract_context.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:async';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/visitor.dart';
 import 'package:analyzer/exception/exception.dart';
@@ skipping 64 matching lines @@
 }
 ''');
 
   Source addPackageSource(String packageName, String filePath, String content) {
     packageMap[packageName] = [(newFolder('/pubcache/$packageName'))];
     File file = newFile('/pubcache/$packageName/$filePath', content);
     return file.createSource();
   }
 
   Source addSource(String path, String content, [Uri uri]) {
+    if (path.startsWith('/')) {
+      path = provider.convertPath(path);
+    }
     driver.addFile(path);
     driver.changeFile(path);
     _fileContentOverlay[path] = content;
     return provider.getFile(path).createSource();
   }
 
   Element findElementInUnit(CompilationUnit unit, String name,
       [ElementKind kind]) {
     return findChildElement(unit.element, name, kind);
   }
@@ skipping 81 matching lines @@
   final _ElementVisitorFunction function;
 
   _ElementVisitorFunctionWrapper(this.function);
 
   @override
   visitElement(Element element) {
     function(element);
     super.visitElement(element);
   }
 }