Fixes #254 - Better stacktrace support

test/codegen/expect/expect.js

 dart_library.library('expect', null, /* Imports */[
   "dart_runtime/dart",
   'dart/core'
 ], /* Lazy imports */[
 ], function(exports, dart, core) {
   'use strict';
   let dartx = dart.dartx;
   class Expect extends core.Object {
     static _truncateString(string, start, end, length) {
       if (dart.notNull(end) - dart.notNull(start) > dart.notNull(length)) {
@@ skipping 249 matching lines @@
         }
         return;
       }
 
       Expect._fail(`Expect.throws${msg} fails: Did not throw`);
     }
     static _getMessage(reason) {
       return reason == null ? "" : `, '${reason}'`;
     }
     static _fail(message) {
-      throw new ExpectException(message);
+      dart.throw(new ExpectException(message));
     }
   }
   dart.setSignature(Expect, {
     statics: () => ({
       _truncateString: [core.String, [core.String, core.int, core.int, core.int]],
       _stringDifference: [core.String, [core.String, core.String]],
       equals: [dart.void, [dart.dynamic, dart.dynamic], [core.String]],
       isTrue: [dart.void, [dart.dynamic], [core.String]],
       isFalse: [dart.void, [dart.dynamic], [core.String]],
       isNull: [dart.void, [dart.dynamic], [core.String]],
@@ skipping 51 matching lines @@
   dart.setSignature(AssumeDynamic, {
     constructors: () => ({AssumeDynamic: [AssumeDynamic, []]})
   });
   // Exports:
   exports.Expect = Expect;
   exports.ExpectException = ExpectException;
   exports.NoInline = NoInline;
   exports.TrustTypeAnnotations = TrustTypeAnnotations;
   exports.AssumeDynamic = AssumeDynamic;
 });