linter: discarded_futures: do not report on await-in-synchronous function

When a synchronous function body has an `await` expression, we report
an error. We don't need to _also_ report a lint.

The noted error, `CompileTimeErrorCode.AWAIT_IN_WRONG_CONTEXT`, already
is associated with the "add async" fix. Reporting a lint when an error
is also definitely reported is just double reporting.

Change-Id: If6d260a01cc654c87897480996d3f6f4b61373dd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/429063
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

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

@@ -549,48 +549,6 @@ Future<void> g() async { }
 ''');
   }
 
-  Future<void> test_discardedFuture_awaited() async {
-    await resolveTestCode('''
-void f() {
-  // ignore: await_in_wrong_context
-  await g();
-}
-
-Future<void> g() async { }
-''');
-    await assertHasFix('''
-Future<void> f() async {
-  // ignore: await_in_wrong_context
-  await g();
-}
-
-Future<void> g() async { }
-''');
-  }
-
-  Future<void> test_discardedFuture_awaited_method() async {
-    await resolveTestCode('''
-class C {
-  void f() {
-    // ignore: await_in_wrong_context
-    await g();
-  }
-
-  Future<void> g() async { }
-}
-''');
-    await assertHasFix('''
-class C {
-  Future<void> f() async {
-    // ignore: await_in_wrong_context
-    await g();
-  }
-
-  Future<void> g() async { }
-}
-''');
-  }
-
   Future<void> test_discardedFuture_method() async {
     await resolveTestCode('''
 class C {

pkg/linter/lib/src/rules/discarded_futures.dart

@@ -50,18 +50,9 @@ class _Visitor extends SimpleAstVisitor<void> {
   void visitExpressionStatement(ExpressionStatement node) {
     var expr = node.expression;
     if (expr is AssignmentExpression) return;
-
-    if (_isEnclosedInAsyncFunctionBody(node)) {
-      return;
-    }
-
-    if (expr case AwaitExpression(:var expression)) {
-      expr = expression;
-    }
-
-    if (expr.isAwaitNotRequired) {
-      return;
-    }
+    if (_isEnclosedInAsyncFunctionBody(node)) return;
+    if (expr is AwaitExpression) return;
+    if (expr.isAwaitNotRequired) return;
 
     var type = expr.staticType;
     if (type == null) {

pkg/linter/test/rules/discarded_futures_test.dart

@@ -265,13 +265,15 @@ Future<int> g() async => 0;
     await assertDiagnostics(
       '''
 void f() {
-  // ignore: await_in_wrong_context
   await g();
 }
 
-Future<void> g() async { }
+Future<void> g() async {}
 ''',
-      [lint(55, 1)],
+      [
+        // No lint.
+        error(CompileTimeErrorCode.AWAIT_IN_WRONG_CONTEXT, 13, 5),
+      ],
     );
   }
 
@@ -280,14 +282,16 @@ Future<void> g() async { }
       '''
 class C {
   void f() {
-    // ignore: await_in_wrong_context
     await g();
   }
 
-  Future<void> g() async { }
+  Future<void> g() async {}
 }
 ''',
-      [lint(71, 1)],
+      [
+        // No lint.
+        error(CompileTimeErrorCode.AWAIT_IN_WRONG_CONTEXT, 27, 5),
+      ],
     );
   }