feat(core): provide an error context when an exception happens in an error handler

modules/angular2/src/facade/lang.ts

@@ -9,6 +9,8 @@ export function getTypeNameForDebugging(type: Type): string {
   return type['name'];
 }
 
+export var isDart = false;
+
 export class BaseException extends Error {
   stack;
   constructor(public message?: string, private _originalException?, private _originalStack?,