fixes dart.throw to work for non-objects

lib/runtime/_operations.js

Index: lib/runtime/_operations.js
diff --git a/lib/runtime/_operations.js b/lib/runtime/_operations.js
index 4d964b441b8f3200e9d220d7d93ae5daffec1033..822054efbb0a3632ab7fd5f3059cd27a51ab6832 100644
--- a/lib/runtime/_operations.js
+++ b/lib/runtime/_operations.js
@@ -269,16 +269,21 @@ dart_library.library('dart_runtime/_operations', null, /* Imports */[
   }
   exports.assert = assert;
 
-  let _stack = Symbol('_stack');
-
+  let _stack = new WeakMap();
   function throw_(obj) {
-    obj[_stack] = new Error();
+    if (obj != null && (typeof obj == 'object' || typeof obj == 'function')) {
+      // TODO(jmesserly): couldn't we store the most recent stack in a single
+      // variable? There should only be one active stack trace. That would
+      // allow it to work for things like strings and numbers.

[vsm, 2015/07/23 14:00:04]

I don't think we want this in the general case.  I encounter a lot of wrapped
exceptions (e.g., in async code), where it's really useful to look at the inner
exception's trace.

Another possibility is to box the string, etc., if we encounter a primitive:

obj = Object(obj);

and throw that instead.

[Jennifer Messerly, 2015/07/23 15:51:30]

Sure, but they should be wrapping the stack trace parameter too? Then they'll
have saved the original inner stack trace, and it will all work.
eeep. I suspect that would break a lot of things

+      _stack.set(obj, new Error());
+    }
     throw obj;
   }
   exports.throw = throw_;
 
   function getError(exception) {
-    return exception[_stack] ? exception[_stack] : exception;
+    var stack = _stack.get(exception);
+    return stack !== void 0 ? stack : exception;
   }
 
   // This is a utility function: it is only intended to be called from dev