Roll Observatory packages and add a roll script

packages/when/example/read_json_file.dart

+
+library when.example.read_json_file;
+
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:when/when.dart';
+
+/// Reads and decodes JSON from [path] asynchronously.
+///
+/// If [path] does not exist, returns the result of calling [onAbsent].
+Future readJsonFile(String path, {onAbsent()}) => _readJsonFile(
+    path, onAbsent, (file) => file.exists(), (file) => file.readAsString());
+
+/// Reads and decodes JSON from [path] synchronously.
+///
+/// If [path] does not exist, returns the result of calling [onAbsent].
+readJsonFileSync(String path, {onAbsent()}) => _readJsonFile(
+    path, onAbsent, (file) => file.existsSync(),
+    (file) => file.readAsStringSync());
+
+_readJsonFile(String path, onAbsent(), exists(File file), read(File file)) {
+  var file = new File(path);
+  return when(
+      () => exists(file),
+      onSuccess: (doesExist) => doesExist ?
+          when(() => read(file), onSuccess: JSON.decode) :
+          onAbsent());
+}
+
+main() {
+  var syncJson = readJsonFileSync('foo.json', onAbsent: () => {'foo': 'bar'});
+  print('Sync json: $syncJson');
+  readJsonFile('foo.json', onAbsent: () => {'foo': 'bar'}).then((asyncJson) {
+    print('Async json: $asyncJson');
+  });
+}