Fix tests for missing required SDK libraries on Windows.

I have not tried them on Windows, hopefully it will help.

R=brianwilkerson@google.com

Change-Id: I1446a834f95398979337db4b887e3d76c3ec164f
Reviewed-on: https://dart-review.googlesource.com/48880
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

Author: scheglov

pkg/analyzer/test/src/dart/analysis/driver_test.dart

@@ -7845,7 +7845,7 @@ import 'b.dart';
   }
 
   test_missingDartLibrary_async() async {
-    provider.getFile(MockSdk.FULL_URI_MAP['dart:async']).delete();
+    provider.getFile(_p(MockSdk.FULL_URI_MAP['dart:async'])).delete();
     addTestFile('class C {}');
 
     ErrorsResult result = await driver.getErrors(testFile);
@@ -7856,7 +7856,7 @@ import 'b.dart';
   }
 
   test_missingDartLibrary_core() async {
-    provider.getFile(MockSdk.FULL_URI_MAP['dart:core']).delete();
+    provider.getFile(_p(MockSdk.FULL_URI_MAP['dart:core'])).delete();
     addTestFile('class C {}');
 
     ErrorsResult result = await driver.getErrors(testFile);