Roll Observatory packages and add a roll script

when/lib/when.dart

-
-library when;
-
-import 'dart:async';
-
-/// Registers callbacks on the result of a [callback], which may or may not be
-/// a [Future].
-///
-/// If [callback] returns a future, any of [onSuccess], [onError], or
-/// [onComplete] that are provided are registered on the future,
-/// and the resulting future is returned.
-///
-/// Otherwise, if [callback] did not throw, if [onSuccess] is provided, it is
-/// called with the with the result of [callback], and the return value of
-/// [onSuccess] is captured.
-///
-/// Otherwise, if [onError] was provided, it is called.  It can take either
-/// just an error, or a stack trace as well.  If [onError] was not provided,
-/// the error is not caught.
-///
-/// [onComplete] is then called synchronously.
-///
-/// The captured value is then returned.
-when(callback, {onSuccess(result), onError, onComplete}) {
-  var result, hasResult = false;
-
-  try {
-    result = callback();
-    hasResult = true;
-  } catch (e, s) {
-    if (onError != null) {
-      if (onError is _Unary) {
-        onError(e);
-      } else if (onError is _Binary) {
-        onError(e, s);
-      } else {
-        throw new ArgumentError(
-            '"onError" must accept 1 or 2 arguments: $onError');
-      }
-    } else {
-      rethrow;
-    }
-  } finally {
-    if (result is Future) {
-      if (onSuccess != null) {
-        result = result.then(onSuccess);
-      }
-      if (onError != null) {
-        result = result.catchError(onError);
-      }
-      if (onComplete != null) {
-        result = result.whenComplete(onComplete);
-      }
-    } else {
-      if (hasResult) {
-        if (onSuccess != null) {
-          result = onSuccess(result);
-        }
-      }
-      if (onComplete != null) onComplete();
-    }
-  }
-
-  return result;
-}
-
-typedef _Unary(x);
-typedef _Binary(x, y);