Add <eval-link> - a custom element for evaluating an expression.

runtime/bin/vmservice/client/lib/src/elements/eval_link.dart

+// Copyright (c) 2013, 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 eval_link_element;
+
+import 'package:observatory/service.dart';
+import 'package:polymer/polymer.dart';
+
+@CustomTag('eval-link')
+class EvalLinkElement extends PolymerElement {
+  EvalLinkElement.created() : super.created();
+
+  @observable bool busy = false;
+  @published var callback = null;
+  @published String expr = '';
+  @published ServiceObject result = null;
+
+  void evalNow(var a, var b, var c) {
+    if (busy) {
+      return;
+    }
+    if (callback != null) {
+      busy = true;
+      result = null;
+      callback(expr).then((ServiceObject obj) {
+          result = obj;
+          busy = false;

[Cutch, 2014/03/31 23:21:51]

What if callback completes with an error? The element will forever be busy
because then is never fired. Add a whenComplete or catchError clause?

[turnidge, 2014/04/16 19:59:00]

Done, good catch.

+        });
+    }
+  }
+}