Add a working command line debugger to Observatory.

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

 <link rel="import" href="../../../../packages/polymer/polymer.html">
 <link rel="import" href="observatory_element.html">
 
 <polymer-element name="script-inset" extends="observatory-element">
   <template>
     <style>
       .sourceInset {
       }
       .sourceBox {
         background-color: #f5f5f5;
         border: 1px solid #ccc;
         padding: 10px;
         overflow-y: auto;
       }
       .sourceTable {
         display: table;
       }
       .sourceRow {
         display: table-row;
       }
       .sourceItem, .sourceItemCurrent {
         display: table-cell;
         vertical-align: top;
         font: 400 14px consolas, courier, monospace;
         line-height: 125%;
         white-space: pre;
       }
-      .sourceItemCurrent {
+      .currentLine {
+        background-color: #fff;
+      }
+      .currentCol {
         background-color: #6cf;
       }
       .hitsNone, .hitsNotExecuted, .hitsExecuted {
         display: table-cell;
         vertical-align: top;
         font: 400 14px consolas, courier, monospace;
         min-width: 32px;
         text-align: right;
         color: #a8a8a8;
       }
@@ skipping 23 matching lines @@
                   <template if="{{ line.hits == 0 }}">
                     <div class="hitsNotExecuted">{{ line.line }}</div>
                   </template>
                   <template if="{{ line.hits > 0 }}">
                     <div class="hitsExecuted">{{ line.line }}</div>
                   </template>
 
                   <div class="sourceItem">&nbsp;</div>
 
                   <template if="{{ line.line == currentLine }}">
-                    <div class="sourceItemCurrent">{{line.text}}</div>
+                    <div class="sourceItem"><span class="currentLine">{{
+                          clip(line.text,0,currentCol
+                        )}}</span><span class="currentCol">{{
+                          clip(line.text,currentCol,currentCol+1)
+                        }}</span><span class="currentLine">{{
+                          clip(line.text,currentCol+1)
+                        }}</span></div>
                   </template>
                   <template if="{{ line.line != currentLine }}">
                     <div class="sourceItem">{{line.text}}</div>
                   </template>
               </div>
               </template>
 
               <template if="{{ line == null }}">
                 <breakpoint-toggle line="{{ null }}"></breakpoint-toggle>
                 <div class="sourceItem">&nbsp;</div>
@@ skipping 78 matching lines @@
         <div class="resolvedBreakpoint">
           <a on-click="{{ toggleBreakpoint }}">B</a>
         </div>
       </template>
     </template> <!-- line != null -->
 
   </template>
 </polymer-element>
 
 <script type="application/dart" src="script_inset.dart"></script>