Fix tests under Windows (TBR)

pkg/analyzer_plugin/test/support/abstract_single_unit.dart

Index: pkg/analyzer_plugin/test/support/abstract_single_unit.dart
diff --git a/pkg/analyzer_plugin/test/support/abstract_single_unit.dart b/pkg/analyzer_plugin/test/support/abstract_single_unit.dart
index 7faabe2470382c0b719e30a6da6bc3c526e6302a..7a603b58dbd4f0136f285c8372a6a60be74fdb7c 100644
--- a/pkg/analyzer_plugin/test/support/abstract_single_unit.dart
+++ b/pkg/analyzer_plugin/test/support/abstract_single_unit.dart
@@ -19,7 +19,7 @@ class AbstractSingleUnitTest extends AbstractContextTest {
   bool verifyNoTestUnitErrors = true;
 
   String testCode;
-  String testFile = '/test.dart';
+  String testFile;
   Source testSource;
   CompilationUnit testUnit;
   CompilationUnitElement testUnitElement;
@@ -112,4 +112,10 @@ class AbstractSingleUnitTest extends AbstractContextTest {
     testUnitElement = testUnit.element;
     testLibraryElement = testUnitElement.library;
   }
+
+  @override
+  void setUp() {
+    super.setUp();
+    testFile = provider.convertPath('/test.dart');
+  }
 }