Refactor Future constructors.

tests/lib/async/future_constructor_test.dart

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library future_test;
+
+import "package:expect/expect.dart";
+import 'dart:async';
+import 'dart:isolate';
+
+main() {
+  compare(func) {
+    // Compare the results of the following two futures.
+    Future f1 = new Future(func);
+    Future f2 = new Future.value().then((_) => func());
+    f2.catchError((_){});  // I'll get the error later.
+    f1.then((v1) { f2.then((v2) { Expect.equals(v1, v2); }); },
+            onError: (e1) {
+              f2.then((_) { Expect.fail("Expected error"); },
+                      onError: (e2) {
+                         Expect.equals(e1, e2);
+                      });
+            });
+  }
+  Future val = new Future.value(42);
+  Future err1 = new Future.error("Error")..catchError((_){});
+  compare(() => 42);
+  compare(() => val);
+  compare(() { throw "Flif"; });
+  compare(() => err1);
+  bool hasExecuted = false;
+  compare(() {
+    hasExecuted = true;
+    return 499;
+  });
+  Expect.isFalse(hasExecuted);
+}
+