Add a working command line debugger to Observatory.

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

Index: runtime/observatory/lib/src/elements/script_inset.dart
diff --git a/runtime/observatory/lib/src/elements/script_inset.dart b/runtime/observatory/lib/src/elements/script_inset.dart
index f129f71bf503b7b037b3326e7d42f4b490f36a9b..aef3fe3dde57d321ee2f8a107482a897802cf8a6 100644
--- a/runtime/observatory/lib/src/elements/script_inset.dart
+++ b/runtime/observatory/lib/src/elements/script_inset.dart
@@ -23,6 +23,7 @@ class ScriptInsetElement extends ObservatoryElement {
   @published int endPos;
 
   @observable int currentLine;
+  @observable int currentCol;
   @observable int startLine;
   @observable int endLine;
   @observable bool linesReady = false;
@@ -34,6 +35,16 @@ class ScriptInsetElement extends ObservatoryElement {
     return 'line-$line';
   }
 
+  String clip(String line, int start, [int limit]) {
+    try {
+      return line.substring(start, limit);
+    } catch (_) {
+      // NOTE(turnidge): Sometimes polymer updates give us garbage
+      // starts and limits during page updates.
+      return "OOB";
+    }
+  }
+
   MutationObserver _observer;
 
   void _scrollToCurrentPos() {
@@ -108,6 +119,9 @@ class ScriptInsetElement extends ObservatoryElement {
     currentLine = (currentPos != null
                    ? script.tokenToLine(currentPos)
                    : null);
+    currentCol = (currentPos != null
+                  ? (script.tokenToCol(currentPos) - 1)  // make this 0-based.
+                  : null);
     endLine = (endPos != null
                ? script.tokenToLine(endPos)
                : script.lines.length);