[generators] Improve a test.

R=mstarzinger@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2056283002
Cr-Commit-Position: refs/heads/master@{#36895}

test/mjsunit/regress/regress-618657.js

@@ -5,6 +5,10 @@
 // Flags: --ignition --ignition-generators --ignition-filter=-foo
 
 function* foo() { yield 42 }
-var g = foo();
+function* goo() { yield 42 }
+var f = foo();
+var g = goo();
+assertEquals(42, f.next().value);
 assertEquals(42, g.next().value);
+assertEquals(true, f.next().done);
 assertEquals(true, g.next().done);