Revert "Issue 42797. Understand in legacy libraries that a function returning Never (in Null Safety library) never returns."

This reverts commit 6bba750.

Reason for revert: Starts reporting new hints in front-end libraries. Breaks bots. Will clean-up and reland.

Original change's description:
> Issue 42797. Understand in legacy libraries that a function returning Never (in Null Safety library) never returns.
> 
> R=​brianwilkerson@google.com
> 
> Bug: #42797
> Change-Id: I14e50c231935a4025d33762843f5a54a1ee2154c
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/155500
> Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

TBR=scheglov@google.com,brianwilkerson@google.com

Change-Id: I99e845d9bd1d9c0882fecc8d72d18d5582454461
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: #42797
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/155540
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

Author: scheglov

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

@@ -91,7 +91,7 @@ typedef WorkToWaitAfterComputingResult = Future<void> Function(String path);
 /// TODO(scheglov) Clean up the list of implicitly analyzed files.
 class AnalysisDriver implements AnalysisDriverGeneric {
   /// The version of data format, should be incremented on every format change.
-  static const int DATA_VERSION = 107;
+  static const int DATA_VERSION = 106;
 
   /// The length of the list returned by [_computeDeclaredVariablesSignature].
   static const int _declaredVariablesSignatureLength = 4;

pkg/analyzer/lib/src/dart/resolver/exit_detector.dart

@@ -6,7 +6,6 @@ import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/dart/element/element.dart';
-import 'package:analyzer/src/dart/element/type.dart';
 
 /// Instances of the class `ExitDetector` determine whether the visited AST node
 /// is guaranteed to terminate by executing a `return` statement, `throw`
@@ -424,7 +423,7 @@ class ExitDetector extends GeneralizingAstVisitor<bool> {
       }
     }
     Element element = node.methodName.staticElement;
-    if (_elementExits(element)) {
+    if (element != null && element.hasAlwaysThrows) {
       return true;
     }
     return _nodeExits(node.argumentList);
@@ -661,14 +660,4 @@ class ExitDetector extends GeneralizingAstVisitor<bool> {
   static bool exits(AstNode node) {
     return ExitDetector()._nodeExits(node);
   }
-
-  static bool _elementExits(Element element) {
-    if (element is ExecutableElement) {
-      var declaration = element.declaration;
-      return declaration.hasAlwaysThrows ||
-          identical(declaration.returnType, NeverTypeImpl.instance);
-    }
-
-    return false;
-  }
 }

pkg/analyzer/test/src/diagnostics/missing_return_test.dart

@@ -7,12 +7,10 @@ import 'package:analyzer/src/test_utilities/package_mixin.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import '../dart/resolution/driver_resolution.dart';
-import '../dart/resolution/with_null_safety_mixin.dart';
 
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(MissingReturnTest);
-    defineReflectiveTests(MissingReturnWithNullSafetyTest);
   });
 }
 
@@ -215,23 +213,3 @@ class B extends A {
     ]);
   }
 }
-
-@reflectiveTest
-class MissingReturnWithNullSafetyTest extends DriverResolutionTest
-    with WithNullSafetyMixin {
-  test_returnNever() async {
-    newFile('/test/lib/a.dart', content: r'''
-Never foo() {
-  throw 0;
-}
-''');
-    await assertNoErrorsInCode(r'''
-// @dart = 2.8
-import 'a.dart';
-
-int f() {
-  foo();
-}
-''');
-  }
-}