Use AbsolutePathContext in Folder.contains().

pkg/analysis_server/lib/src/context_manager.dart

Index: pkg/analysis_server/lib/src/context_manager.dart
diff --git a/pkg/analysis_server/lib/src/context_manager.dart b/pkg/analysis_server/lib/src/context_manager.dart
index 0b1a2d7c4a84ad4574615ed9d972f4d032c30c39..430b75c1a23d351677eaa340eda10f315f709244 100644
--- a/pkg/analysis_server/lib/src/context_manager.dart
+++ b/pkg/analysis_server/lib/src/context_manager.dart
@@ -343,11 +343,6 @@ class ContextManagerImpl implements ContextManager {
   final ResourceProvider resourceProvider;
 
   /**
-   * The context used to work with file system paths.
-   */
-  pathos.Context pathContext;
-
-  /**
    * The context used to work with absolute file system paths.
    *
    * TODO(scheglov) remove [pathContext].
@@ -355,6 +350,11 @@ class ContextManagerImpl implements ContextManager {
   AbsolutePathContext absolutePathContext;
 
   /**
+   * The context used to work with file system paths.
+   */
+  pathos.Context pathContext;
+
+  /**
    * The list of excluded paths (folders and files) most recently passed to
    * [setRoots].
    */
@@ -417,8 +417,8 @@ class ContextManagerImpl implements ContextManager {
 
   ContextManagerImpl(this.resourceProvider, this.packageResolverProvider,
       this._packageMapProvider, this._instrumentationService) {
+    absolutePathContext = resourceProvider.absolutePathContext;
     pathContext = resourceProvider.pathContext;
-    absolutePathContext = new AbsolutePathContext(pathContext.separator);
   }
 
   @override