dart2js: AsyncRewriter improvements

The general purpose rewritten code has several shortcomings

[ ] Some expressions are needlessly assigned to temporaries. This is usually when await is a subexpression.
```
$async$temp1 = $async$self;
$async$temp2 = J;
...
$async$temp2.$eq$($async$result, $async$self._field)
```
We should never put $async$self (aka captured 'this') in a temporary. The JavaScript could come with a side-table of property accesses that are insensitive to evaluation order, to prevent other cases like J.
[ ] Return statements are transformed into code that uses the state-machine to goto a single return case. These should rather be generated in-place. Doing so would often result in smaller minified code and fewer case statements and might allow the elimination of the returnValue variable. To do this well, the transformer will require awareness of when the continuation is a return, since the return can be after a join point or outside a loop.
[ ] Receiver type checks and argument type checks are compiled to if (test) return e where e always throws. These should be left as-is and not be transformed into async return code.
[ ] If there is no exception control flow, the rethrow code can be folded into the error code test, eliminating the handler and currentError variables and the case label.
[ ] Straight-line code might not need the loop (with the above changes it might never use 'break' to reach the switch twice)

Example:

    foo: function() {
      var $async$goto = 0, $async$completer = P.Completer_Completer$sync(), $async$returnValue, $async$handler = 2, $async$currentError;
      var $async$foo = P._wrapJsFunctionForAsync(function($async$errorCode, $async$result) {
        if ($async$errorCode === 1) {
          $async$currentError = $async$result;
          $async$goto = $async$handler;
        }
        while (true)
          switch ($async$goto) {
            case 0:
              // Function start
              $async$goto = 3;
              return P._asyncAwait(R.bar(), $async$foo);
            case 3:
              // returning from await.
              $async$returnValue = $async$result;
              // goto return
              $async$goto = 1;
              break;
            case 1:
              // return
              return P._asyncReturn($async$returnValue, $async$completer);
            case 2:
              // rethrow
              return P._asyncRethrow($async$currentError, $async$completer);
          }
      });
      return P._asyncStart($async$foo, $async$completer);
    },

    foo: function() {
      var $async$goto = 0, $async$completer = P.Completer_Completer$sync();
      var $async$foo = P._wrapJsFunctionForAsync(function($async$errorCode, $async$result) {
        if ($async$errorCode === 1)
          return P._asyncRethrow($async$result, $async$completer);
        switch ($async$goto) {
          case 0:
            // Function start
            $async$goto = 1;
            return P._asyncAwait(R.bar(), $async$foo);
          case 1:
            // returning from await.
            // return
            return P._asyncReturn($async$result, $async$completer);
        }
      });
      return P._asyncStart($async$foo, $async$completer);
    },

dart-lang / sdk

dart2js: AsyncRewriter improvements #29532