Handle "as" in kernel with ir.AsExpression case.

pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart

 // Copyright (c) 2016, 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.
 
 import 'package:js_runtime/shared/embedded_names.dart';
 import 'package:kernel/ast.dart' as ir;
 
 import '../constants/expressions.dart';
 import '../common.dart';
 import '../common/names.dart';
@@ skipping 300 matching lines @@
   ir.Procedure get checkConcurrentModificationError =>
       kernel.functions[_backend.helpers.checkConcurrentModificationError];
 
   TypeMask get checkConcurrentModificationErrorReturnType =>
       TypeMaskFactory.inferredReturnTypeForElement(
           _backend.helpers.checkConcurrentModificationError, _compiler);
 
   ir.Procedure get assertHelper =>
       kernel.functions[_backend.helpers.assertHelper];
 
+  TypeMask get throwTypeErrorType => TypeMaskFactory
+      .inferredReturnTypeForElement(_backend.helpers.throwTypeError, _compiler);
+
   TypeMask get assertHelperReturnType => TypeMaskFactory
       .inferredReturnTypeForElement(_backend.helpers.assertHelper, _compiler);
 
   ir.Procedure get assertTest => kernel.functions[_backend.helpers.assertTest];
 
   TypeMask get assertTestReturnType => TypeMaskFactory
       .inferredReturnTypeForElement(_backend.helpers.assertTest, _compiler);
 
   ir.Procedure get assertThrow =>
       kernel.functions[_backend.helpers.assertThrow];
@@ skipping 430 matching lines @@
     astAdapter.reporter.internalError(
         CURRENT_ELEMENT_SPANNABLE, "Unexpected constant target: $element.");
     return null;
   }
 
   @override
   ConstantExpression visitStringLiteral(ir.StringLiteral node) {
     return new StringConstantExpression(node.value);
   }
 }