Fixes #254 - Better stacktrace support

lib/runtime/dart/_js_mirrors.js

 dart_library.library('dart/_js_mirrors', null, /* Imports */[
   "dart_runtime/dart",
   'dart/_internal',
   'dart/core',
   'dart/mirrors'
 ], /* Lazy imports */[
 ], function(exports, dart, _internal, core, mirrors) {
   'use strict';
   let dartx = dart.dartx;
   function getName(symbol) {
     return _internal.Symbol.getName(dart.as(symbol, _internal.Symbol));
   }
   dart.fn(getName, core.String, [core.Symbol]);
   function getSymbol(name, library) {
-    return dart.throw_(new core.UnimplementedError("MirrorSystem.getSymbol unimplemented"));
+    return dart.throw(new core.UnimplementedError("MirrorSystem.getSymbol unimplemented"));
   }
   dart.fn(getSymbol, core.Symbol, [dart.dynamic, dart.dynamic]);
   dart.defineLazyProperties(exports, {
     get currentJsMirrorSystem() {
-      return dart.throw_(new core.UnimplementedError("MirrorSystem.currentJsMirrorSystem unimplemented"));
+      return dart.throw(new core.UnimplementedError("MirrorSystem.currentJsMirrorSystem unimplemented"));
     }
   });
   function reflect(reflectee) {
     return new JsInstanceMirror._(reflectee);
   }
   dart.fn(reflect, mirrors.InstanceMirror, [dart.dynamic]);
   function reflectType(key) {
     return new JsClassMirror._(key);
   }
   dart.fn(reflectType, mirrors.TypeMirror, [core.Type]);
@@ skipping 167 matching lines @@
   exports.getName = getName;
   exports.getSymbol = getSymbol;
   exports.reflect = reflect;
   exports.reflectType = reflectType;
   exports.JsInstanceMirror = JsInstanceMirror;
   exports.JsClassMirror = JsClassMirror;
   exports.JsTypeMirror = JsTypeMirror;
   exports.JsParameterMirror = JsParameterMirror;
   exports.JsMethodMirror = JsMethodMirror;
 });