Remove BlockExpression from the Kernel language.

lib/binary/ast_from_binary.dart

Index: lib/binary/ast_from_binary.dart
diff --git a/lib/binary/ast_from_binary.dart b/lib/binary/ast_from_binary.dart
index 40a178b01eac1d2088269a360532ca096d93538f..bbad877591cf1b1e3a6bb434a5f6e64d7c7c3eba 100644
--- a/lib/binary/ast_from_binary.dart
+++ b/lib/binary/ast_from_binary.dart
@@ -660,10 +660,6 @@ class BinaryBuilder {
         var body = readExpression();
         variableStack.length = stackHeight;
         return new Let(variable, body);
-      case Tag.BlockExpression:
-        var body = readBlock();
-        var expression = readExpression();
-        return new BlockExpression(body, expression);
       default:
         throw fail('Invalid expression tag: $tag');
     }