Fix test tests under windows (TBR)

pkg/analyzer_plugin/test/utilities/completion/completion_contributor_util.dart

Index: pkg/analyzer_plugin/test/utilities/completion/completion_contributor_util.dart
diff --git a/pkg/analyzer_plugin/test/utilities/completion/completion_contributor_util.dart b/pkg/analyzer_plugin/test/utilities/completion/completion_contributor_util.dart
index 24e115082c613dd8b1e28a349781b55ee8330a65..ddceaf7804064e10f0625f007bc1f5ff829dad67 100644
--- a/pkg/analyzer_plugin/test/utilities/completion/completion_contributor_util.dart
+++ b/pkg/analyzer_plugin/test/utilities/completion/completion_contributor_util.dart
@@ -27,7 +27,7 @@ int suggestionComparator(CompletionSuggestion s1, CompletionSuggestion s2) {
 
 abstract class DartCompletionContributorTest extends AbstractContextTest {
   static const String _UNCHECKED = '__UNCHECKED__';
-  String testFile = '/completionTest.dart';
+  String testFile;
   Source testSource;
   int completionOffset;
   int replacementOffset;
@@ -564,6 +564,7 @@ abstract class DartCompletionContributorTest extends AbstractContextTest {
   @override
   void setUp() {
     super.setUp();
+    testFile = provider.convertPath('/completionTest.dart');
     contributor = createContributor();
   }
 }