Always evaluate e in (e is T)

runtime/vm/parser.cc

Index: runtime/vm/parser.cc
===================================================================
--- runtime/vm/parser.cc	(revision 33288)
+++ runtime/vm/parser.cc	(working copy)
@@ -7608,11 +7608,14 @@
         if (((op_kind == Token::kIS) || (op_kind == Token::kISNOT) ||
              (op_kind == Token::kAS)) &&
             type.IsMalformedOrMalbounded()) {
-          // Note that a type error is thrown even if the tested value is null
-          // in a type test or in a type cast.
-          // TODO(hausner): We drop the left operand. We need to
-          // evaluate it in case there are side effects.
-          left_operand = ThrowTypeError(type_pos, type);
+          // Note that a type error is thrown in a type test or in
+          // a type cast even if the tested value is null.
+          // We need to evaluate the left operand for potential
+          // side effects.
+          LetNode* let = new LetNode(left_operand->token_pos());
+          let->AddNode(left_operand);
+          let->AddNode(ThrowTypeError(type_pos, type));
+          left_operand = let;
           break;  // Type checks and casts can't be chained.
         }
       }