diff --git a/pkgs/test/CHANGELOG.md b/pkgs/test/CHANGELOG.md
index dab1ac9..a16e171 100644
--- a/pkgs/test/CHANGELOG.md
+++ b/pkgs/test/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 1.15.5 (Backport)
+
+* Fix `spawnHybridUri` to respect language versioning of the spawned uri.
+
 ## 1.15.4
 
 * Allow analyzer 0.40.x.
diff --git a/pkgs/test/lib/src/runner/browser/platform.dart b/pkgs/test/lib/src/runner/browser/platform.dart
index 32162bc..cd8347c 100644
--- a/pkgs/test/lib/src/runner/browser/platform.dart
+++ b/pkgs/test/lib/src/runner/browser/platform.dart
@@ -5,7 +5,6 @@
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
-import 'dart:isolate';
 
 import 'package:async/async.dart';
 import 'package:http_multi_server/http_multi_server.dart';
@@ -29,6 +28,7 @@
 import 'package:test_core/src/runner/runner_suite.dart'; // ignore: implementation_imports
 import 'package:test_core/src/runner/suite.dart'; // ignore: implementation_imports
 import 'package:test_core/src/util/io.dart'; // ignore: implementation_imports
+import 'package:test_core/src/util/package_config.dart'; // ignore: implementation_imports
 import 'package:test_core/src/util/stack_trace_mapper.dart'; // ignore: implementation_imports
 import 'package:web_socket_channel/web_socket_channel.dart';
 import 'package:yaml/yaml.dart';
@@ -48,12 +48,13 @@
   /// the working directory.
   static Future<BrowserPlatform> start({String root}) async {
     var server = shelf_io.IOServer(await HttpMultiServer.loopback(0));
+    var packageConfig = await currentPackageConfig;
     return BrowserPlatform._(
         server,
         Configuration.current,
-        p.fromUri(await Isolate.resolvePackageUri(
+        p.fromUri(packageConfig.resolve(
             Uri.parse('package:test/src/runner/browser/static/favicon.ico'))),
-        p.fromUri(await Isolate.resolvePackageUri(Uri.parse(
+        p.fromUri(packageConfig.resolve(Uri.parse(
             'package:test/src/runner/browser/static/default.html.tpl'))),
         root: root);
   }
diff --git a/pkgs/test/lib/src/runner/node/platform.dart b/pkgs/test/lib/src/runner/node/platform.dart
index a9601ed..671e7ad 100644
--- a/pkgs/test/lib/src/runner/node/platform.dart
+++ b/pkgs/test/lib/src/runner/node/platform.dart
@@ -21,6 +21,7 @@
 import 'package:test_core/src/runner/runner_suite.dart'; // ignore: implementation_imports
 import 'package:test_core/src/runner/suite.dart'; // ignore: implementation_imports
 import 'package:test_core/src/util/io.dart'; // ignore: implementation_imports
+import 'package:test_core/src/util/package_config.dart'; // ignore: implementation_imports
 import 'package:test_core/src/util/stack_trace_mapper.dart'; // ignore: implementation_imports
 import 'package:test_core/src/runner/application_exception.dart'; // ignore: implementation_imports
 import 'package:test_core/src/runner/compiler_pool.dart'; // ignore: implementation_imports
diff --git a/pkgs/test/lib/src/util/package_map.dart b/pkgs/test/lib/src/util/package_map.dart
index 948b6a6..6b30afa 100644
--- a/pkgs/test/lib/src/util/package_map.dart
+++ b/pkgs/test/lib/src/util/package_map.dart
@@ -2,15 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'dart:isolate';
-
 import 'package:package_config/package_config.dart';
 
-/// The [PackageConfig] parsed from the current isolates package config file.
-final Future<PackageConfig> currentPackageConfig = () async {
-  return loadPackageConfigUri(await Isolate.packageConfig);
-}();
-
 /// Adds methods to convert to a package map on [PackageConfig].
 extension PackageMap on PackageConfig {
   /// A package map exactly matching the current package config
diff --git a/pkgs/test/pubspec.yaml b/pkgs/test/pubspec.yaml
index ebacee2..e66201a 100644
--- a/pkgs/test/pubspec.yaml
+++ b/pkgs/test/pubspec.yaml
@@ -1,5 +1,5 @@
 name: test
-version: 1.15.4
+version: 1.15.5
 description: A full featured library for writing and running Dart tests.
 homepage: https://github.com/dart-lang/test/blob/master/pkgs/test
 
@@ -32,8 +32,8 @@
   webkit_inspection_protocol: ">=0.5.0 <0.8.0"
   yaml: ^2.0.0
   # Use an exact version until the test_api and test_core package are stable.
-  test_api: 0.2.18
-  test_core: 0.3.11+1
+  test_api: 0.2.18+1
+  test_core: 0.3.11+2
 
 dev_dependencies:
   fake_async: ^1.0.0
diff --git a/pkgs/test/test/runner/hybrid_test.dart b/pkgs/test/test/runner/hybrid_test.dart
index 7c1b0dd..827611a 100644
--- a/pkgs/test/test/runner/hybrid_test.dart
+++ b/pkgs/test/test/runner/hybrid_test.dart
@@ -7,6 +7,7 @@
 import 'dart:io';
 import 'dart:isolate';
 
+import 'package:package_config/package_config.dart';
 import 'package:path/path.dart' as p;
 import 'package:test_descriptor/test_descriptor.dart' as d;
 
@@ -394,6 +395,33 @@
           ]));
       await test.shouldExit(0);
     });
+
+    test('can opt out of null safety', () async {
+      expect(spawnHybridCode('''
+        // @dart=2.9
+        import "package:stream_channel/stream_channel.dart";
+
+        // Would cause an error in null safety mode.
+        int x;
+
+        void hybridMain(StreamChannel channel) {
+          channel.sink..add(1)..add(2)..add(3)..close();
+        }
+      ''').stream.toList(), completion(equals([1, 2, 3])));
+    });
+
+    test('opts in to null safety by default', () async {
+      expect(spawnHybridCode('''
+        import "package:stream_channel/stream_channel.dart";
+
+        // Use some null safety syntax
+        int? x;
+
+        void hybridMain(StreamChannel channel) {
+          channel.sink..add(1)..add(2)..add(3)..close();
+        }
+      ''').stream.toList(), completion(equals([1, 2, 3])));
+    });
   });
 }
 
@@ -584,4 +612,107 @@
     expect(spawnHybridUri('non existent file').stream.first,
         throwsA(TypeMatcher<Exception>()));
   });
+
+  test('can opt out of nnbd via language version comments', () async {
+    await d.file('test.dart', '''
+        import "package:test/test.dart";
+
+        void main() {
+          test("hybrid emits numbers", () {
+            expect(spawnHybridUri("hybrid.dart").stream.toList(),
+                completion(equals([1, 2, 3])));
+          });
+        }
+      ''').create();
+
+    await d.file('hybrid.dart', '''
+        // @dart=2.9
+        import "package:stream_channel/stream_channel.dart";
+
+        // Would fail if null safety were enabled.
+        int x;
+
+        void hybridMain(StreamChannel channel) {
+          channel.sink..add(1)..add(2)..add(3)..close();
+        }
+      ''').create();
+
+    var test = await runTest(['test.dart', ...arguments]);
+    expect(test.stdout,
+        containsInOrder(['+0: hybrid emits numbers', '+1: All tests passed!']));
+    await test.shouldExit(0);
+  });
+
+  test('can opt in to nnbd via language version comments', () async {
+    await d.file('test.dart', '''
+        import "package:test/test.dart";
+
+        void main() {
+          test("hybrid emits numbers", () {
+            expect(spawnHybridUri("hybrid.dart").stream.toList(),
+                completion(equals([1, 2, 3])));
+          });
+        }
+      ''').create();
+
+    await d.file('hybrid.dart', '''
+        // @dart=2.12
+        import "package:stream_channel/stream_channel.dart";
+
+        // Use some null safety syntax to confirm we are opted in.
+        int? x;
+
+        void hybridMain(StreamChannel channel) {
+          channel.sink..add(1)..add(2)..add(3)..close();
+        }
+      ''').create();
+
+    var test = await runTest(['test.dart', ...arguments]);
+    expect(test.stdout,
+        containsInOrder(['+0: hybrid emits numbers', '+1: All tests passed!']));
+    await test.shouldExit(0);
+  });
+
+  test('the default language version is used for hybrid code', () async {
+    await d.file('test.dart', '''
+        import "package:test/test.dart";
+
+        void main() {
+          test("hybrid emits numbers", () {
+            expect(spawnHybridUri("hybrid.dart").stream.toList(),
+                completion(equals([1, 2, 3])));
+          });
+        }
+      ''').create();
+
+    await d.file('hybrid.dart', '''
+        import "package:stream_channel/stream_channel.dart";
+
+        // Would fail if null safety were enabled.
+        int x;
+
+        void hybridMain(StreamChannel channel) {
+          channel.sink..add(1)..add(2)..add(3)..close();
+        }
+      ''').create();
+
+    // Adds the sandbox dir as a new package to the existing config,
+    // opting it out.
+    var originalPackageConfig =
+        await loadPackageConfigUri(await Isolate.packageConfig);
+    var extraPackage = Package('_test', Uri.file('${d.sandbox}/'),
+        languageVersion: LanguageVersion(2, 9));
+    var newConfig = PackageConfig([
+      ...originalPackageConfig.packages,
+      extraPackage,
+    ], extraData: originalPackageConfig.extraData);
+    await d.dir('.dart_tool').create();
+    await savePackageConfig(newConfig, Directory(d.sandbox));
+
+    var test = await runTest(['test.dart', ...arguments],
+        packageConfig: p.join(d.sandbox, '.dart_tool', 'package_config.json'));
+    expect(test.stdout,
+        containsInOrder(['+0: hybrid emits numbers', '+1: All tests passed!']));
+    await test.shouldExit(0);
+  });
 }
diff --git a/pkgs/test_api/CHANGELOG.md b/pkgs/test_api/CHANGELOG.md
index eb65ce8..232a62a 100644
--- a/pkgs/test_api/CHANGELOG.md
+++ b/pkgs/test_api/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 0.2.18+1 (Backport)
+
+* Fix `spawnHybridUri` to respect language versioning of the spawned uri.
+
 ## 0.2.18
 
 * Update to `matcher` version `0.12.9`.
diff --git a/pkgs/test_api/lib/src/frontend/spawn_hybrid.dart b/pkgs/test_api/lib/src/frontend/spawn_hybrid.dart
index 1b0cde2..cab7ff2 100644
--- a/pkgs/test_api/lib/src/frontend/spawn_hybrid.dart
+++ b/pkgs/test_api/lib/src/frontend/spawn_hybrid.dart
@@ -6,11 +6,9 @@
 import 'dart:convert';
 
 import 'package:async/async.dart';
-import 'package:path/path.dart' as p;
 import 'package:stream_channel/stream_channel.dart';
 
 import '../../test_api.dart';
-import '../backend/invoker.dart';
 import '../util/remote_exception.dart';
 import '../utils.dart';
 
@@ -92,52 +90,13 @@
 /// **Note**: If you use this API, be sure to add a dependency on the
 /// **`stream_channel` package, since you're using its API as well!
 StreamChannel spawnHybridUri(uri, {Object message, bool stayAlive = false}) {
-  Uri parsedUrl;
-  if (uri is Uri) {
-    parsedUrl = uri;
-  } else if (uri is String) {
-    parsedUrl = Uri.parse(uri);
-  } else {
+  if (uri is String) {
+    // Ensure that it can be parsed as a uri.
+    Uri.parse(uri);
+  } else if (uri is! Uri) {
     throw ArgumentError.value(uri, 'uri', 'must be a Uri or a String.');
   }
-
-  String absoluteUri;
-  if (parsedUrl.scheme.isEmpty) {
-    var isRootRelative = parsedUrl.path.startsWith('/');
-
-    // If we're running in a browser context, the working directory is already
-    // relative to the test file, whereas on the VM the working directory is the
-    // root of the package.
-    if (p.style == p.Style.url) {
-      if (isRootRelative) {
-        // A root-relative URL is interpreted as relative to the package root,
-        // which means placing it beneath the URL secret.
-        var secret = Uri.encodeComponent(Uri.base.pathSegments[0]);
-        absoluteUri = p.absolute('/$secret$parsedUrl');
-        print('Uri.base: ${Uri.base}');
-        print('absoluteUri: ${absoluteUri}');
-      } else {
-        absoluteUri = p.absolute(parsedUrl.toString());
-      }
-    } else {
-      if (isRootRelative) {
-        // We assume that the current path is the package root. `pub run`
-        // enforces this currently, but at some point it would probably be good
-        // to pass in an explicit root.
-        absoluteUri = p.url
-            .join(p.toUri(p.current).toString(), parsedUrl.path.substring(1));
-      } else {
-        var suitePath = Invoker.current.liveTest.suite.path;
-        absoluteUri = p.url.join(
-            p.url.dirname(p.toUri(p.absolute(suitePath)).toString()),
-            parsedUrl.toString());
-      }
-    }
-  } else {
-    absoluteUri = uri.toString();
-  }
-
-  return _spawn(absoluteUri, message, stayAlive: stayAlive);
+  return _spawn(uri.toString(), message, stayAlive: stayAlive);
 }
 
 /// Spawns a VM isolate that runs the given [dartCode], which is loaded as the
diff --git a/pkgs/test_api/pubspec.yaml b/pkgs/test_api/pubspec.yaml
index 40d4171..186a3a2 100644
--- a/pkgs/test_api/pubspec.yaml
+++ b/pkgs/test_api/pubspec.yaml
@@ -1,5 +1,5 @@
 name: test_api
-version: 0.2.18
+version: 0.2.18+1
 description: A library for writing Dart tests.
 homepage: https://github.com/dart-lang/test/blob/master/pkgs/test_api
 
diff --git a/pkgs/test_core/CHANGELOG.md b/pkgs/test_core/CHANGELOG.md
index 8c3b03f..7e27b7d 100644
--- a/pkgs/test_core/CHANGELOG.md
+++ b/pkgs/test_core/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 0.3.11+2 (Backport)
+
+* Fix `spawnHybridUri` to respect language versioning of the spawned uri.
+
 ## 0.3.11+1
 
 * Allow analyzer 0.40.x.
diff --git a/pkgs/test_core/lib/src/runner/runner_test.dart b/pkgs/test_core/lib/src/runner/runner_test.dart
index 0dc6229..d910cac 100644
--- a/pkgs/test_core/lib/src/runner/runner_test.dart
+++ b/pkgs/test_core/lib/src/runner/runner_test.dart
@@ -71,7 +71,7 @@
             // When we kill the isolate that the test lives in, that will close
             // this virtual channel and cause the spawned isolate to close as
             // well.
-            spawnHybridUri(message['url'] as String, message['message'])
+            spawnHybridUri(message['url'] as String, message['message'], suite)
                 .pipe(testChannel.virtualChannel(message['channel'] as int));
             break;
         }
diff --git a/pkgs/test_core/lib/src/runner/spawn_hybrid.dart b/pkgs/test_core/lib/src/runner/spawn_hybrid.dart
index 1db6769..76e448a 100644
--- a/pkgs/test_core/lib/src/runner/spawn_hybrid.dart
+++ b/pkgs/test_core/lib/src/runner/spawn_hybrid.dart
@@ -3,14 +3,19 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
+import 'dart:io';
 import 'dart:isolate';
 
+import 'package:analyzer/dart/analysis/utilities.dart';
 import 'package:async/async.dart';
+import 'package:path/path.dart' as p;
 import 'package:stream_channel/isolate_channel.dart';
 import 'package:stream_channel/stream_channel.dart';
 
 import '../util/dart.dart' as dart;
+import '../util/package_config.dart';
 
+import 'package:test_api/src/backend/suite.dart'; // ignore: implementation_imports
 import 'package:test_api/src/util/remote_exception.dart'; // ignore: implementation_imports
 
 /// Spawns a hybrid isolate from [url] with the given [message], and returns a
@@ -19,12 +24,23 @@
 /// This connects the main isolate to the hybrid isolate, whereas
 /// `lib/src/frontend/spawn_hybrid.dart` connects the test isolate to the main
 /// isolate.
-StreamChannel spawnHybridUri(String url, Object message) {
+///
+/// If [uri] is relative, it will be interpreted relative to the `file:` URL
+/// for [suite]. If it's root-relative (that is, if it begins with `/`) it will
+/// be interpreted relative to the root of the package (the directory that
+/// contains `pubspec.yaml`, *not* the `test/` directory). If it's a `package:`
+/// URL, it will be resolved using the current package's dependency
+/// constellation.
+StreamChannel /*!*/ spawnHybridUri(
+    String url, Object /*?*/ message, Suite suite) {
+  url = _normalizeUrl(url, suite);
   return StreamChannelCompleter.fromFuture(() async {
     var port = ReceivePort();
     var onExitPort = ReceivePort();
     try {
       var code = '''
+        ${await _languageVersionCommentFor(url)}
+
         import "package:test_core/src/runner/hybrid_listener.dart";
 
         import "${url.replaceAll(r'$', '%24')}" as lib;
@@ -64,3 +80,75 @@
     }
   }());
 }
+
+/// Normalizes [url] to an absolute url, or returns it as is if it has a
+/// scheme.
+///
+/// Follows the rules for relatives/absolute paths outlit
+String _normalizeUrl(String url, Suite suite) {
+  final parsedUri = Uri.parse(url);
+
+  if (parsedUri.scheme.isEmpty) {
+    var isRootRelative = parsedUri.path.startsWith('/');
+
+    if (isRootRelative) {
+      // We assume that the current path is the package root. `pub run`
+      // enforces this currently, but at some point it would probably be good
+      // to pass in an explicit root.
+      return p.url
+          .join(p.toUri(p.current).toString(), parsedUri.path.substring(1));
+    } else {
+      var suitePath = suite.path;
+      return p.url.join(
+          p.url.dirname(p.toUri(p.absolute(suitePath)).toString()),
+          parsedUri.toString());
+    }
+  } else {
+    return url;
+  }
+}
+
+/// Computes the a language version comment for the library at [uri].
+///
+/// If there is a language version comment in the file, that is returned.
+///
+/// Otherwise a comment representing the default version from the
+/// [currentPackageConfig] is returned.
+///
+/// If no default language version is known (data: uri for instance), then
+/// an empty string is returned.
+Future<String> _languageVersionCommentFor(String url) async {
+  var parsedUri = Uri.parse(url);
+
+  // Returns the explicit language version comment if one exists.
+  var result = parseString(
+      content: await _readUri(parsedUri),
+      path: parsedUri.path,
+      throwIfDiagnostics: false);
+  var languageVersionComment = result.unit.languageVersionToken?.value();
+  if (languageVersionComment != null) return languageVersionComment.toString();
+
+  // Returns the default language version for the package if one exists.
+  if (parsedUri.scheme.isEmpty || parsedUri.scheme == 'file') {
+    var packageConfig = await currentPackageConfig;
+    var package = packageConfig.packageOf(parsedUri);
+    var version = package?.languageVersion;
+    if (version != null) return '// @dart=${version}';
+  }
+
+  // Fall back on no language comment.
+  return '';
+}
+
+Future<String> _readUri(Uri uri) async {
+  switch (uri.scheme) {
+    case '':
+    case 'file':
+      return File.fromUri(uri).readAsString();
+    case 'data':
+      return uri.data.contentAsString();
+    default:
+      throw ArgumentError.value(uri, 'uri',
+          'Only data and file uris (as well as relative paths) are supported');
+  }
+}
diff --git a/pkgs/test_core/lib/src/util/package_config.dart b/pkgs/test_core/lib/src/util/package_config.dart
new file mode 100644
index 0000000..b3743f8
--- /dev/null
+++ b/pkgs/test_core/lib/src/util/package_config.dart
@@ -0,0 +1,14 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+//
+// @dart=2.7
+
+import 'dart:isolate';
+
+import 'package:package_config/package_config.dart';
+
+/// The [PackageConfig] parsed from the current isolates package config file.
+final Future<PackageConfig> currentPackageConfig = () async {
+  return loadPackageConfigUri(await Isolate.packageConfig);
+}();
diff --git a/pkgs/test_core/pubspec.yaml b/pkgs/test_core/pubspec.yaml
index 1375468..7e26f8c 100644
--- a/pkgs/test_core/pubspec.yaml
+++ b/pkgs/test_core/pubspec.yaml
@@ -1,5 +1,5 @@
 name: test_core
-version: 0.3.11+1
+version: 0.3.11+2
 description: A basic library for writing tests and running them on the VM.
 homepage: https://github.com/dart-lang/test/blob/master/pkgs/test_core
 
@@ -30,7 +30,7 @@
   # matcher is tightly constrained by test_api
   matcher: any
   # Use an exact version until the test_api package is stable.
-  test_api: 0.2.18
+  test_api: 0.2.18+1
 
 dependency_overrides:
   test_api: