Fixes #254 - Better stacktrace support

lib/runtime/_operations.js

Index: lib/runtime/_operations.js
diff --git a/lib/runtime/_operations.js b/lib/runtime/_operations.js
index 6f9294edfe6a23ecfbcd9e8454f66cec48ac1458..64f6b97bab6974f5232420a7ac9102540b24af3b 100644
--- a/lib/runtime/_operations.js
+++ b/lib/runtime/_operations.js
@@ -269,12 +269,28 @@ dart_library.library('dart_runtime/_operations', null, /* Imports */[
   }
   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;