Update MockSdk to be more consistent with the actual SDK, switch x_Driver tests to it.

pkg/analyzer/test/generated/compile_time_error_code_test.dart

Index: pkg/analyzer/test/generated/compile_time_error_code_test.dart
diff --git a/pkg/analyzer/test/generated/compile_time_error_code_test.dart b/pkg/analyzer/test/generated/compile_time_error_code_test.dart
index 73b66dd95b80497a57807a2ad45567366caae8ac..aabfe67874b9ac678e08f89f649eace3e5a688f1 100644
--- a/pkg/analyzer/test/generated/compile_time_error_code_test.dart
+++ b/pkg/analyzer/test/generated/compile_time_error_code_test.dart
@@ -2773,19 +2773,6 @@ class B extends A {
     verify([source]);
   }
 
-  test_importInternalLibrary_js_helper() async {
-    Source source = addSource("import 'dart:_js_helper';");
-    // Note, in these error cases we may generate an UNUSED_IMPORT hint, while
-    // we could prevent the hint from being generated by testing the import
-    // directive for the error, this is such a minor corner case that we don't
-    // think we should add the additional computation time to figure out such
-    // cases.
-    await computeAnalysisResult(source);
-    assertErrors(source,
-        [CompileTimeErrorCode.IMPORT_INTERNAL_LIBRARY, HintCode.UNUSED_IMPORT]);
-    verify([source]);
-  }
-
   test_importOfNonLibrary() async {
     Source source = addSource(r'''
 library lib;