Added support to backtrace from botton of stack to debugger protocol

src/d8.js

Index: src/d8.js
===================================================================
--- src/d8.js	(revision 1840)
+++ src/d8.js	(working copy)
@@ -498,9 +498,26 @@
 DebugRequest.prototype.backtraceCommandToJSONRequest_ = function(args) {
   // Build a backtrace request from the text command.
   var request = this.createRequest('backtrace');
+  
+  // Default is to show top 10 frames.
+  request.arguments = {};
+  request.arguments.fromFrame = 0;
+  request.arguments.toFrame = 10;
+
   args = args.split(/\s*[ ]+\s*/g);
-  if (args.length == 2) {
-    request.arguments = {};
+  if (args.length == 1 && args[0].length > 0) {
+    var frameCount = parseInt(args[0]);
+    if (frameCount > 0) {
+      // Show top frames.
+      request.arguments.fromFrame = 0;
+      request.arguments.toFrame = frameCount;
+    } else {
+      // Show bottom frames.
+      request.arguments.fromFrame = 0;
+      request.arguments.toFrame = -frameCount;
+      request.arguments.bottom = true;
+    }
+  } else if (args.length == 2) {
     var fromFrame = parseInt(args[0]);
     var toFrame = parseInt(args[1]);
     if (isNaN(fromFrame) || fromFrame < 0) {
@@ -513,9 +530,13 @@
       throw new Error('Invalid arguments start frame cannot be larger ' +
                       'than end frame.');
     }
+    // Show frame range.
     request.arguments.fromFrame = fromFrame;
     request.arguments.toFrame = toFrame + 1;
+  } else if (args.length > 2) {
+    throw new Error('Invalid backtrace arguments.');
   }
+
   return request.toJSONProtocol();
 };
 
@@ -755,7 +776,7 @@
   print('  break on function: location is #<id>#');
   print('  break on script position: location is name:line[:column]');
   print('clear <breakpoint #>');
-  print('backtrace [from frame #] [to frame #]]');
+  print('backtrace [n] | [-n] | [from to]');
   print('frame <frame #>');
   print('step [in | next | out| min [step count]]');
   print('print <expression>');