Fix another windows-specific test bug (TBR)

pkg/analyzer_plugin/test/src/utilities/completion/completion_target_test.dart

Index: pkg/analyzer_plugin/test/src/utilities/completion/completion_target_test.dart
diff --git a/pkg/analyzer_plugin/test/src/utilities/completion/completion_target_test.dart b/pkg/analyzer_plugin/test/src/utilities/completion/completion_target_test.dart
index 711491ad058c07a6780a81568ac3e839c466e359..92a29a85af732982d0344835fbf71e828d73721b 100644
--- a/pkg/analyzer_plugin/test/src/utilities/completion/completion_target_test.dart
+++ b/pkg/analyzer_plugin/test/src/utilities/completion/completion_target_test.dart
@@ -33,7 +33,7 @@ class CompletionTargetTest extends AbstractContextTest {
     expect(nextOffset, equals(-1), reason: 'too many ^');
     content = content.substring(0, completionOffset) +
         content.substring(completionOffset + 1);
-    testSource = addSource('/test.dart', content);
+    testSource = addSource(provider.convertPath('/test.dart'), content);
     AnalysisResult result = await driver.getResult(testSource.fullName);
     target = new CompletionTarget.forOffset(result.unit, completionOffset);
   }