lib/json/json.dart has the following code

class _JsonParser {
  static List<int> tokens;
  _JsonParser(String json)
      : json = json,
        length = json.length {
    if (tokens != null) return;

    // Use a list as jump-table. It is faster than switch and if.
    tokens = new List<int>(LAST_ASCII + 1);
    tokens[TAB] = WHITESPACE;
    tokens[NEW_LINE] = WHITESPACE;
    ... // 25 assignments total
    tokens[CHAR_F] = FALSE_LITERAL;
  }

It compiles to:
_JsonParser$1: function(json) {
  if (!($._JsonParser_tokens == null))
    return;
  $._JsonParser_tokens = $.List_List(126);
  $.indexSet($._JsonParser_tokens, 9, 32);
  $.indexSet($._JsonParser_tokens, 10, 32);
  ...
  $.indexSet($._JsonParser_tokens, 102, 102);
}

In the body of the function, global _JsonParser.tokens is an alias for the list.

If I assign the list to a local (also called tokens), the compiler does a much better job, eliminating all range checks:

_JsonParser$1: function(json) {
  var tokens;
  if (!($._JsonParser_tokens == null))
    return;
  tokens = $.List_List(126);
  tokens[9] = 32;
  tokens[10] = 32;
  ...
  tokens[102] = 102;
  this._JsonParser_tokens = tokens;
}

What would be required to enable the compiler to optimize the original code to the same as the hand-optimized code?