Fixes #254 - Better stacktrace support

test/codegen/expect/try_catch.js

 dart_library.library('try_catch', null, /* Imports */[
   "dart_runtime/dart",
   'dart/core'
 ], /* Lazy imports */[
 ], function(exports, dart, core) {
   'use strict';
   let dartx = dart.dartx;
   function foo() {
     try {
-      throw "hi there";
+      dart.throw("hi there");
     } catch (e$) {
       if (dart.is(e$, core.String)) {
         let e = e$;
         let t = dart.stackTrace(e);
       } else {
         let e = e$;
         let t = dart.stackTrace(e);
         throw e;
       }
     }
 
   }
   dart.fn(foo);
   function bar() {
     try {
-      throw "hi there";
+      dart.throw("hi there");
     } catch (e$) {
       let e = e$;
       let t = dart.stackTrace(e);
     }
 
   }
   dart.fn(bar);
   function baz() {
     try {
-      throw "finally only";
+      dart.throw("finally only");
     } finally {
       return true;
     }
   }
   dart.fn(baz);
   function qux() {
     try {
-      throw "on only";
+      dart.throw("on only");
     } catch (e) {
       if (dart.is(e, core.String)) {
         let t = dart.stackTrace(e);
         throw e;
       } else
         throw e;
     }
 
   }
   dart.fn(qux);
   function wub() {
     try {
-      throw "on without exception parameter";
+      dart.throw("on without exception parameter");
     } catch (e) {
       if (dart.is(e, core.String)) {
       } else
         throw e;
     }
 
   }
   dart.fn(wub);
   function main() {
     foo();
     bar();
     baz();
     qux();
     wub();
   }
   dart.fn(main);
   // Exports:
   exports.foo = foo;
   exports.bar = bar;
   exports.baz = baz;
   exports.qux = qux;
   exports.wub = wub;
   exports.main = main;
 });