fix #27766, allow implicit casts from dynamic to composite types

pkg/analyzer/lib/src/task/strong/checker.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.checker;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 1070 matching lines @@
       // This cast is (probably) due to our different treatment of dynamic.
       // It may be more likely to fail at runtime.
       if (from is InterfaceType) {
         // For class types, we'd like to allow non-generic down casts, e.g.,
         // Iterable<T> to List<T>.  The intuition here is that raw (generic)
         // casts are problematic, and we should complain about those.
         var typeArgs = from.typeArguments;
         downCastComposite =
             typeArgs.isEmpty || typeArgs.any((t) => t.isDynamic);
       } else {
-        downCastComposite = true;
+        downCastComposite = !from.isDynamic;
       }
     }
 
     var parent = expr.parent;
     ErrorCode errorCode;
     if (downCastComposite) {
       errorCode = StrongModeCode.DOWN_CAST_COMPOSITE;
     } else if (from.isDynamic) {
       errorCode = StrongModeCode.DYNAMIC_CAST;
     } else if (parent is VariableDeclaration && parent.initializer == expr) {
@@ skipping 375 matching lines @@
     var visited = new Set<InterfaceType>();
     do {
       visited.add(current);
       current.mixins.reversed.forEach(
           (m) => _checkIndividualOverridesFromClass(node, m, seen, true));
       _checkIndividualOverridesFromClass(node, current.superclass, seen, true);
       current = current.superclass;
     } while (!current.isObject && !visited.contains(current));
   }
 }