diff --git a/pkg/analyzer/test/src/summary/top_level_inference_test.dart b/pkg/analyzer/test/src/summary/top_level_inference_test.dart
index b9bf27d..4038ead 100644
--- a/pkg/analyzer/test/src/summary/top_level_inference_test.dart
+++ b/pkg/analyzer/test/src/summary/top_level_inference_test.dart
@@ -2,14 +2,10 @@
 // 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 'package:analyzer/dart/analysis/results.dart';
 import 'package:analyzer/dart/element/element.dart';
-import 'package:analyzer/src/dart/analysis/experiments.dart';
 import 'package:analyzer/src/error/codes.dart';
-import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
-import '../dart/analysis/base.dart';
 import '../dart/resolution/context_collection_resolution.dart';
 import 'element_text.dart';
 
@@ -17,7 +13,6 @@
   defineReflectiveSuite(() {
     defineReflectiveTests(TopLevelInferenceTest);
     defineReflectiveTests(TopLevelInferenceErrorsTest);
-    defineReflectiveTests(TopLevelInferenceTestWithSpread);
 //    defineReflectiveTests(ApplyCheckElementTextReplacements);
   });
 }
@@ -30,7 +25,8 @@
 }
 
 @reflectiveTest
-class TopLevelInferenceErrorsTest extends PubPackageResolutionTest {
+class TopLevelInferenceErrorsTest extends PubPackageResolutionTest
+    with WithNullSafetyMixin {
   test_initializer_additive() async {
     await _assertErrorOnlyLeft(['+', '-']);
   }
@@ -185,7 +181,7 @@
 
   test_initializer_ifNull() async {
     await assertNoErrorsInCode('''
-var a = 1;
+int? a = 1;
 var t = a ?? 2;
 ''');
   }
@@ -216,8 +212,8 @@
 
   test_initializer_methodInvocation_function() async {
     await assertNoErrorsInCode('''
-int f1() => null;
-T f2<T>() => null;
+int f1() => 0;
+T f2<T>() => throw 0;
 var t1 = f1();
 var t2 = f2();
 var t3 = f2<int>();
@@ -227,8 +223,8 @@
   test_initializer_methodInvocation_method() async {
     await assertNoErrorsInCode('''
 class A {
-  int m1() => null;
-  T m2<T>() => null;
+  int m1() => 0;
+  T m2<T>() => throw 0;
 }
 var a = new A();
 var t1 = a.m1();
@@ -303,17 +299,17 @@
   test_override_conflictFieldType() async {
     await assertErrorsInCode('''
 abstract class A {
-  int aaa;
+  int aaa = 0;
 }
 abstract class B {
-  String aaa;
+  String aaa = '0';
 }
 class C implements A, B {
   var aaa;
 }
 ''', [
-      error(CompileTimeErrorCode.INVALID_OVERRIDE, 99, 3),
-      error(CompileTimeErrorCode.INVALID_OVERRIDE, 99, 3),
+      error(CompileTimeErrorCode.INVALID_OVERRIDE, 109, 3),
+      error(CompileTimeErrorCode.INVALID_OVERRIDE, 109, 3),
     ]);
   }
 
@@ -344,7 +340,8 @@
 }
 
 @reflectiveTest
-class TopLevelInferenceTest extends BaseAnalysisDriverTest {
+class TopLevelInferenceTest extends PubPackageResolutionTest
+    with WithNullSafetyMixin {
   test_initializer_additive() async {
     var library = await _encodeDecodeLibrary(r'''
 var vPlusIntInt = 1 + 2;
@@ -638,7 +635,7 @@
   }
 
   test_initializer_extractProperty_explicitlyTyped_differentLibraryCycle() async {
-    newFile('/test/lib/a.dart', content: r'''
+    newFile('$testPackageLibPath/a.dart', content: r'''
 class C {
   int f = 0;
 }
@@ -669,7 +666,7 @@
   }
 
   test_initializer_extractProperty_explicitlyTyped_sameLibraryCycle() async {
-    newFile('/test/lib/a.dart', content: r'''
+    newFile('$testPackageLibPath/a.dart', content: r'''
 import 'test.dart'; // just do make it part of the library cycle
 class C {
   int f = 0;
@@ -686,7 +683,7 @@
   }
 
   test_initializer_extractProperty_implicitlyTyped_differentLibraryCycle() async {
-    newFile('/test/lib/a.dart', content: r'''
+    newFile('$testPackageLibPath/a.dart', content: r'''
 class C {
   var f = 0;
 }
@@ -717,7 +714,7 @@
   }
 
   test_initializer_extractProperty_implicitlyTyped_sameLibraryCycle() async {
-    newFile('/test/lib/a.dart', content: r'''
+    newFile('$testPackageLibPath/a.dart', content: r'''
 import 'test.dart'; // just do make it part of the library cycle
 class C {
   var f = 0;
@@ -2157,6 +2154,34 @@
   double foo(int x);
 }
 abstract class C implements A, B {
+  Never foo/*error: overrideNoCombinedSuperSignature*/(dynamic x);
+}
+''');
+  }
+
+  test_method_error_noCombinedSuperSignature2_legacy() async {
+    var library = await _encodeDecodeLibrary(r'''
+// @dart = 2.9
+abstract class A {
+  int foo(int x);
+}
+
+abstract class B {
+  double foo(int x);
+}
+
+abstract class C implements A, B {
+  Never foo(x);
+}
+''');
+    checkElementText(library, r'''
+abstract class A {
+  int foo(int x);
+}
+abstract class B {
+  double foo(int x);
+}
+abstract class C implements A, B {
   Null foo/*error: overrideNoCombinedSuperSignature*/(dynamic x);
 }
 ''');
@@ -2590,7 +2615,7 @@
   }
 
   test_method_OK_single_private_linkThroughOtherLibraryOfCycle() async {
-    newFile('/test/lib/other.dart', content: r'''
+    newFile('$testPackageLibPath/other.dart', content: r'''
 import 'test.dart';
 class B extends A2 {}
 ''');
@@ -2685,21 +2710,9 @@
   }
 
   Future<LibraryElement> _encodeDecodeLibrary(String text) async {
-    String path = convertPath('/test/lib/test.dart');
-    newFile(path, content: text);
-    UnitElementResult result = await driver.getUnitElement(path);
+    newFile(testFilePath, content: text);
+    var analysisSession = contextFor(testFilePath).currentSession;
+    var result = await analysisSession.getUnitElement(testFilePath);
     return result.element.library /*!*/;
   }
 }
-
-@reflectiveTest
-class TopLevelInferenceTestWithSpread extends TopLevelInferenceTest {
-  @override
-  List<String> get enabledExperiments => [EnableString.spread_collections];
-
-  @override
-  @failingTest
-  test_initializer_literal_map_untyped_empty() async {
-    fail('times out.');
-  }
-}
diff --git a/runtime/vm/compiler/frontend/kernel_binary_flowgraph.cc b/runtime/vm/compiler/frontend/kernel_binary_flowgraph.cc
index 54435dc..d577091 100644
--- a/runtime/vm/compiler/frontend/kernel_binary_flowgraph.cc
+++ b/runtime/vm/compiler/frontend/kernel_binary_flowgraph.cc
@@ -4886,86 +4886,100 @@
   // function and token position.
   Function& function = Function::ZoneHandle(Z);
 
-  // NOTE: This is not TokenPosition in the general sense!
-  function = I->LookupClosureFunction(parsed_function()->function(), position);
+  {
+    SafepointReadRwLocker ml(thread(),
+                             thread()->isolate_group()->program_lock());
+    // NOTE: This is not TokenPosition in the general sense!
+    function =
+        I->LookupClosureFunction(parsed_function()->function(), position);
+  }
+
   if (function.IsNull()) {
-    for (intptr_t i = 0; i < scopes()->function_scopes.length(); ++i) {
-      if (scopes()->function_scopes[i].kernel_offset != offset) {
-        continue;
-      }
-
-      const String* name;
-      if (declaration) {
-        name = &H.DartSymbolObfuscate(name_index);
-      } else {
-        name = &Symbols::AnonymousClosure();
-      }
-      // NOTE: This is not TokenPosition in the general sense!
-      if (!closure_owner_.IsNull()) {
-        function = Function::NewClosureFunctionWithKind(
-            FunctionLayout::kClosureFunction, *name,
-            parsed_function()->function(), position, closure_owner_);
-      } else {
-        function = Function::NewClosureFunction(
-            *name, parsed_function()->function(), position);
-      }
-
-      function.set_is_debuggable(function_node_helper.dart_async_marker_ ==
-                                 FunctionNodeHelper::kSync);
-      switch (function_node_helper.dart_async_marker_) {
-        case FunctionNodeHelper::kSyncStar:
-          function.set_modifier(FunctionLayout::kSyncGen);
-          break;
-        case FunctionNodeHelper::kAsync:
-          function.set_modifier(FunctionLayout::kAsync);
-          function.set_is_inlinable(!FLAG_causal_async_stacks);
-          break;
-        case FunctionNodeHelper::kAsyncStar:
-          function.set_modifier(FunctionLayout::kAsyncGen);
-          function.set_is_inlinable(!FLAG_causal_async_stacks);
-          break;
-        default:
-          // no special modifier
-          break;
-      }
-      function.set_is_generated_body(function_node_helper.async_marker_ ==
-                                     FunctionNodeHelper::kSyncYielding);
-      // sync* functions contain two nested synthetic functions, the first of
-      // which (sync_op_gen) is a regular sync function so we need to manually
-      // label it generated:
-      if (function.parent_function() != Function::null()) {
-        const auto& parent = Function::Handle(function.parent_function());
-        if (parent.IsSyncGenerator()) {
-          function.set_is_generated_body(true);
+    SafepointWriteRwLocker ml(thread(),
+                              thread()->isolate_group()->program_lock());
+    // NOTE: This is not TokenPosition in the general sense!
+    function =
+        I->LookupClosureFunction(parsed_function()->function(), position);
+    if (function.IsNull()) {
+      for (intptr_t i = 0; i < scopes()->function_scopes.length(); ++i) {
+        if (scopes()->function_scopes[i].kernel_offset != offset) {
+          continue;
         }
+
+        const String* name;
+        if (declaration) {
+          name = &H.DartSymbolObfuscate(name_index);
+        } else {
+          name = &Symbols::AnonymousClosure();
+        }
+        // NOTE: This is not TokenPosition in the general sense!
+        if (!closure_owner_.IsNull()) {
+          function = Function::NewClosureFunctionWithKind(
+              FunctionLayout::kClosureFunction, *name,
+              parsed_function()->function(), position, closure_owner_);
+        } else {
+          function = Function::NewClosureFunction(
+              *name, parsed_function()->function(), position);
+        }
+
+        function.set_is_debuggable(function_node_helper.dart_async_marker_ ==
+                                   FunctionNodeHelper::kSync);
+        switch (function_node_helper.dart_async_marker_) {
+          case FunctionNodeHelper::kSyncStar:
+            function.set_modifier(FunctionLayout::kSyncGen);
+            break;
+          case FunctionNodeHelper::kAsync:
+            function.set_modifier(FunctionLayout::kAsync);
+            function.set_is_inlinable(!FLAG_causal_async_stacks);
+            break;
+          case FunctionNodeHelper::kAsyncStar:
+            function.set_modifier(FunctionLayout::kAsyncGen);
+            function.set_is_inlinable(!FLAG_causal_async_stacks);
+            break;
+          default:
+            // no special modifier
+            break;
+        }
+        function.set_is_generated_body(function_node_helper.async_marker_ ==
+                                       FunctionNodeHelper::kSyncYielding);
+        // sync* functions contain two nested synthetic functions, the first of
+        // which (sync_op_gen) is a regular sync function so we need to manually
+        // label it generated:
+        if (function.parent_function() != Function::null()) {
+          const auto& parent = Function::Handle(function.parent_function());
+          if (parent.IsSyncGenerator()) {
+            function.set_is_generated_body(true);
+          }
+        }
+        // Note: Is..() methods use the modifiers set above, so order matters.
+        if (function.IsAsyncClosure() || function.IsAsyncGenClosure()) {
+          function.set_is_inlinable(!FLAG_causal_async_stacks &&
+                                    !FLAG_lazy_async_stacks);
+        }
+
+        function.set_end_token_pos(function_node_helper.end_position_);
+        LocalScope* scope = scopes()->function_scopes[i].scope;
+        const ContextScope& context_scope = ContextScope::Handle(
+            Z, scope->PreserveOuterScope(flow_graph_builder_->context_depth_));
+        function.set_context_scope(context_scope);
+        function.set_kernel_offset(offset);
+        type_translator_.SetupFunctionParameters(Class::Handle(Z), function,
+                                                 false,  // is_method
+                                                 true,   // is_closure
+                                                 &function_node_helper);
+        // type_translator_.SetupUnboxingInfoMetadata is not called here at the
+        // moment because closures do not have unboxed parameters and return
+        // value
+        function_node_helper.ReadUntilExcluding(FunctionNodeHelper::kEnd);
+
+        // Finalize function type.
+        Type& signature_type = Type::Handle(Z, function.SignatureType());
+        signature_type ^= ClassFinalizer::FinalizeType(signature_type);
+        function.SetSignatureType(signature_type);
+
+        I->AddClosureFunction(function);
+        break;
       }
-      // Note: Is..() methods use the modifiers set above, so order matters.
-      if (function.IsAsyncClosure() || function.IsAsyncGenClosure()) {
-        function.set_is_inlinable(!FLAG_causal_async_stacks &&
-                                  !FLAG_lazy_async_stacks);
-      }
-
-      function.set_end_token_pos(function_node_helper.end_position_);
-      LocalScope* scope = scopes()->function_scopes[i].scope;
-      const ContextScope& context_scope = ContextScope::Handle(
-          Z, scope->PreserveOuterScope(flow_graph_builder_->context_depth_));
-      function.set_context_scope(context_scope);
-      function.set_kernel_offset(offset);
-      type_translator_.SetupFunctionParameters(Class::Handle(Z), function,
-                                               false,  // is_method
-                                               true,   // is_closure
-                                               &function_node_helper);
-      // type_translator_.SetupUnboxingInfoMetadata is not called here at the
-      // moment because closures do not have unboxed parameters and return value
-      function_node_helper.ReadUntilExcluding(FunctionNodeHelper::kEnd);
-
-      // Finalize function type.
-      Type& signature_type = Type::Handle(Z, function.SignatureType());
-      signature_type ^= ClassFinalizer::FinalizeType(signature_type);
-      function.SetSignatureType(signature_type);
-
-      I->AddClosureFunction(function);
-      break;
     }
   }
 
diff --git a/tools/VERSION b/tools/VERSION
index e227f33..857b325 100644
--- a/tools/VERSION
+++ b/tools/VERSION
@@ -27,5 +27,5 @@
 MAJOR 2
 MINOR 12
 PATCH 0
-PRERELEASE 143
+PRERELEASE 144
 PRERELEASE_PATCH 0
\ No newline at end of file