Allow users to remove evaluation results.

runtime/observatory/lib/src/elements/eval_box.html

 <link rel="import" href="../../../../packages/polymer/polymer.html">
 <link rel="import" href="instance_ref.html">
 <link rel="import" href="error_ref.html">
 <link rel="import" href="observatory_element.html">
 
 <polymer-element name="eval-box" extends="observatory-element">
   <template>
     <style>
       .textbox {
         flex-grow: 1;
@@ skipping 24 matching lines @@
         cursor: pointer;
         white-space: pre-line;
       }
       .historyExpr a:hover {
         background-color: #fff3e3;
       }
       .historyValue {
         display: block;
         padding: 6px 6px;
       }
+      a.boxclose {
+        margin-left: 20px;
+        valign: top;
+        display: block;
+        height: 18px;
+        width: 18px;
+        line-height: 16px;
+        border-radius: 9px;
+        color: black;
+        font-size: 18px;
+        cursor: pointer;
+        text-align: center;
+      }
+      a.boxclose:hover {
+        background: lightgray;
+      }
     </style>
     <form style="display:flex; flex-direction:row; align-items:flex-end">
       <template if="{{ lineMode == '1-line' }}">
         <input class="textbox" type="text" value="{{ text }}">
       </template>
       <template if="{{ lineMode == 'N-line' }}">
         <textarea class="bigtextbox" rows="5" cols="80"
                   value="{{ text }}"></textarea>
       </template>
 
@@ skipping 24 matching lines @@
             </template>
             <template if="{{ result['error'] == null}}">
               <template if="{{ result['value'] != null }}">
                 <any-service-ref ref="{{ result['value'] }}"></any-service-ref>
               </template>
               <template if="{{ result['value'] == null }}">
                 <div style="color:#aaa;cursor:wait;">&lt;pending&gt;</div>
               </template>
             </template>
           </td>
+          <td>
+            <a class="boxclose" on-click="{{ closeItem }}"
+               closeId="{{ result['id'] }}">&times;</a>
+          </td>
         </tr>
       </table>
     </template>
   </template>
 </polymer-element>
 
 <script type="application/dart" src="eval_box.dart"></script>