Fix `LetExpression`

[matthias-springer]

The following code does not compile:

List<int> list;
list[4] = list[3]++;

Generated Java code:

dart.core.List_interface__int __tempVar_4;
int __tempVar_5;
int __tempVar_6;
void __tempVar_7;
dart.core.List_interface__int list = null;
list.operatorAtPut_List__int(4, dart._runtime.helpers.LetExpressionHelper.comma(__tempVar_4 = ((dart.core.List_interface__int) list), dart._runtime.helpers.LetExpressionHelper.comma(__tempVar_5 = 3, dart._runtime.helpers.LetExpressionHelper.comma(__tempVar_6 = __tempVar_4.operatorAt_List__int(__tempVar_5), dart._runtime.helpers.LetExpressionHelper.comma(__tempVar_7 = __tempVar_4.operatorAtPut_List__int(__tempVar_5, (__tempVar_6 + 1)), __tempVar_6)))));

This is because of how we use LetExpressionHelper at the moment.

Note: Dart code can always be rewritten in a way such that the code compiles (using extra temp variables).