Add a scroll bar when the console prints looooooooong lines.

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

 <link rel="import" href="../../../../packages/polymer/polymer.html">
 <link rel="import" href="function_ref.html">
 <link rel="import" href="nav_bar.html">
 <link rel="import" href="eval_link.html">
 <link rel="import" href="observatory_element.html">
 <link rel="import" href="script_inset.html">
 <link rel="import" href="script_ref.html">
 
 <!-- TODO(turnidge): Use core-icon once core_elements work properly in
      devtools -->
@@ skipping 82 matching lines @@
         height: 0px;
         margin: 0px;
         font-size: 1px;
         border-bottom: 1px solid #888;
       }
       core-splitter {
         flex: 0 0 auto;
       }
       .console {
         flex: 1 1 auto;
+        overflow-x: auto;
         overflow-y: auto;
       }
       .commandline {
         flex: 0 0 auto;
       }
     </style>
 
     <div class="container">
       <nav-bar id="navbarDiv" notifyOnPause="{{ false }}">
         <top-nav-menu></top-nav-menu>
@@ skipping 334 matching lines @@
         padding: 5px;
         font: 400 16px consolas, courier, monospace;
         width: 95%;
       }
     </style>
     <input id="textBox" class="textBox" type="text" value="{{ text }}" autofocus>
   </template>
 </polymer-element>
 
 <script type="application/dart" src="debugger.dart"></script>