Analyze SDK with non-nullable experiment enabled, update MockSdk.

Initial:  https://dart-review.googlesource.com/c/sdk/+/134401
Reverted: https://dart-review.googlesource.com/c/sdk/+/134802

Internal presubmit is green.
https://test.corp.google.com/ui#id=OCL:294345942:BASE:297377939:1582738004318:fb4ae9fe

Change-Id: I25e6445bd0241a89850dff3d88ece0edd768053a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/135241
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analysis_server/test/src/services/correction/fix/remove_method_declaration_test.dart

@@ -120,7 +120,7 @@ class B extends A {
 ''');
   }
 
-  /// TODO(scheglov) This test will fail with NNBD SDK.
+  @FailingTest(issue: 'https://github.com/dart-lang/linter/issues/1997')
   Future<void> test_method_toString() async {
     await resolveTestUnit('''
 class A {

pkg/analyzer/lib/src/dart/analysis/feature_set_provider.dart

@@ -7,14 +7,11 @@ import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/context/packages.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:meta/meta.dart';
-import 'package:pub_semver/pub_semver.dart';
 
 class FeatureSetProvider {
   /// This flag will be turned to `true` and inlined when we un-fork SDK,
   /// so that the only SDK is the Null Safe SDK.
-  static const isNullSafetySdk = false;
-
-  static final _preNonNullableVersion = Version(2, 7, 0);
+  static const isNullSafetySdk = true;
 
   final FeatureSet _sdkFeatureSet;
   final Packages _packages;
@@ -58,11 +55,7 @@ class FeatureSetProvider {
     @required FeatureSet packageDefaultFeatureSet,
     @required FeatureSet nonPackageDefaultFeatureSet,
   }) {
-    var sdkFeatureSet = _determineSdkFeatureSet(
-      resourceProvider,
-      sourceFactory,
-      packageDefaultFeatureSet,
-    );
+    var sdkFeatureSet = FeatureSet.fromEnableFlags(['non-nullable']);
 
     return FeatureSetProvider._(
       sdkFeatureSet: sdkFeatureSet,
@@ -71,32 +64,4 @@ class FeatureSetProvider {
       nonPackageDefaultFeatureSet: nonPackageDefaultFeatureSet,
     );
   }
-
-  /// Read `dart:core` file and determine if SDK in non-nullable by default.
-  ///
-  /// If it is, use the [defaultFeatureSet], which might have enabled
-  /// [Feature.non_nullable], so SDK will be parsed and resolved as
-  /// non-nullable.
-  ///
-  /// Otherwise, restrict the SDK language to the maximum known now.
-  static FeatureSet _determineSdkFeatureSet(
-    ResourceProvider resourceProvider,
-    SourceFactory sourceFactory,
-    FeatureSet defaultFeatureSet,
-  ) {
-    var objectSource = sourceFactory.forUri('dart:core/object.dart');
-    if (objectSource == null) {
-      objectSource = sourceFactory.forUri('dart:core');
-    }
-
-    try {
-      var objectFile = resourceProvider.getFile(objectSource.fullName);
-      var objectContent = objectFile.readAsStringSync();
-      if (!objectContent.contains('bool operator ==(Object other)')) {
-        return defaultFeatureSet.restrictToVersion(_preNonNullableVersion);
-      }
-    } catch (_) {}
-
-    return defaultFeatureSet;
-  }
 }