Fixes #254 - Better stacktrace support

lib/runtime/_operations.js

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /* This library defines runtime operations on objects used by the code
  * generator.
  */
 dart_library.library('dart_runtime/_operations', null, /* Imports */[
 ], /* Lazy Imports */[
   'dart/async',
@@ skipping 251 matching lines @@
     }
     return map;
   }
   exports.map = map;
 
   function assert(condition) {
     if (!condition) errors.throwAssertionError();
   }
   exports.assert = assert;
 
-  function throw_(obj) { throw obj; }
-  exports.throw_ = throw_;
+  let _stack = Symbol('_stack');
 
+  function throw_(obj) {
+    obj[_stack] = new Error();
+    throw obj;
+  }
+  exports.throw = throw_;
+
+  function getError(exception) {
+    return exception[_stack] ? exception[_stack] : exception;
+  }
+
+  // This is a utility function: it is only intended to be called from dev tools.

[Jennifer Messerly, 2015/07/14 18:30:33]

long line.

Maybe we need a module of debug utils? It doesn't seem like we want extra code
in our runtime.
Doesn't console.log(error) work pretty well for JS errors?

Side note, this really shows why doing Dart specific stuff is not good for
interop =\. It means the JS tools don't work out of the box for us anymore. I'd
love to get back in the business of having "throw" be "throw"

+  function stackPrint(exception) {
+    var error = getError(exception);
+    console.log(error.stack ? error.stack : 'No stack trace for: ' + error);

[Jennifer Messerly, 2015/07/14 18:30:33]

is this different from _js_helper.getTraceFromException ?

I'm a bit confused by stackPrint being different from stackTrace. IMO,
console.log(stackTrace(error)); seems reasonable to ask folks to write.

+  }
+  exports.stackPrint = stackPrint;
 
   function stackTrace(exception) {
-    return _js_helper.getTraceFromException(exception);
+    var error = getError(exception);
+    return _js_helper.getTraceFromException(error);
   }
   exports.stackTrace = stackTrace;
 
   let _value = Symbol('_value');
   /**
    * Looks up a sequence of [keys] in [map], recursively, and
    * returns the result. If the value is not found, [valueFn] will be called to
    * add it. For example:
    *
    *     let map = new Map();
@@ skipping 101 matching lines @@
     next() {
       let i = this.dartIterator;
       let done = !i.moveNext();
       return { done: done, value: done ? void 0 : i.current };
     }
   }
   exports.JsIterator = JsIterator;
 
 
 });