Check for void in rules subtyping (fixes #25069)

pkg/analyzer/lib/src/task/strong/rules.dart

 // Copyright (c) 2015, 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.
 
 // TODO(jmesserly): this was ported from package:dev_compiler, and needs to be
 // refactored to fit into analyzer.
 library analyzer.src.task.strong.rules;
 
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
@@ skipping 303 matching lines @@
     if (t1 is TypeParameterType) {
       DartType bound = t1.element.bound;
       if (bound == null) return false;
       return isSubTypeOf(bound, t2);
     }
 
     if (t2 is TypeParameterType) {
       return false;
     }
 
+    if (t1.isVoid || t2.isVoid) {
+      return false;
+    }
+
     if (t2.isDartCoreFunction) {
       if (t1 is FunctionType) return true;
       if (t1.element is ClassElement) {
         if ((t1.element as ClassElement).getMethod("call") != null) return true;
       }
     }
 
     // "Traditional" name-based subtype check.
     if (t1 is InterfaceType && t2 is InterfaceType) {
       return _isInterfaceSubTypeOf(t1, t2);
@@ skipping 177 matching lines @@
       {cast: true}) {
     if (rules.isSubTypeOf(rules.getStaticType(e), t)) return true;
     if (cast && rules.getStaticType(e).isDynamic) {
       annotateCastFromDynamic(e, t);
       return true;
     }
     errors.add("$e cannot be typed as $t");
     return false;
   }
 }