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

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

Index: pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
diff --git a/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart b/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
index 6d8943de579119ce2f1695bd85298ff815273a91..07c515c3a0388e23df5a1effbff2cd586e67c69f 100644
--- a/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
+++ b/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
@@ -318,6 +318,9 @@ class KernelAstAdapter {
   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);