PromiseCoerce should deal with an error during accessing "then".

PromiseCource(x) should return a rejected promise when accessing x.then
leads to an error.

BUG=347095
LOG=Y
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/194483002

Patch from Yutaka Hirano <yhirano@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19928 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/promise.js

@@ -227,7 +227,15 @@ PromiseCoerce.table = new $WeakMap;
 
 function PromiseCoerce(constructor, x) {
   if (!IsPromise(x) && IS_SPEC_OBJECT(x)) {
-    var then = x.then;
+    var then;
+    try {
+      then = x.then;
+    } catch(e) {
+      var deferred = %_CallFunction(constructor, PromiseDeferred);
+      PromiseCoerce.table.set(x, deferred.promise);
+      deferred.reject(e);
+      return deferred.promise;
+    }
     if (typeof then === 'function') {
       if (PromiseCoerce.table.has(x)) {
         return PromiseCoerce.table.get(x);