Debugger: add ability to set script source from within OnBeforeCompile.

src/mirror-debugger.js

Index: src/mirror-debugger.js
===================================================================
--- src/mirror-debugger.js	(revision 11040)
+++ src/mirror-debugger.js	(working copy)
@@ -1832,6 +1832,11 @@
 };
 
 
+ScriptMirror.prototype.setSource = function(source) {
+  %DebugSetScriptSource(this.script_, source);
+};
+
+
 ScriptMirror.prototype.lineOffset = function() {
   return this.script_.line_offset;
 };