[analysis_server] Fix isolate plugin test on Windows

Fixes:
  Expected: ['/pkg1']
  Actual: ['C:\\pkg1']
   Which: at location [0] is 'C:\\pkg1' instead of '/pkg1'
Change-Id: I6bd2abaa77eea7193794ece126201958e11639db
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/508040
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>

pkg/analysis_server/test/src/plugin/plugin_isolate_test.dart

@@ -71,7 +71,7 @@ class PluginIsolateTest with ResourceProviderMixin, _ContextRoot {
     expect(roots1[0]['optionsFile'], optionsFile.path);
     expect(sentRequests[1].method, 'analysis.setAnalysisRoots');
     var roots2 = sentRequests[1].params['included'] as List<String>;
-    expect(roots2, ['/pkg1']);
+    expect(roots2, [contextRoot.root.path]);
   }
 
   void test_creation() {