Fix error handling in dart2js async-await

tests/language/sync_generator3_test.dart

Index: tests/language/sync_generator3_test.dart
diff --git a/tests/language/sync_generator3_test.dart b/tests/language/sync_generator3_test.dart
index d84275e5d7efeb77702064c47efca1ef287a359f..a993a2792f2ab6164d1816e31eea8fee50856b67 100644
--- a/tests/language/sync_generator3_test.dart
+++ b/tests/language/sync_generator3_test.dart
@@ -34,14 +34,12 @@ g() sync* {
 }
 
 test2() {
-  var s;
-  try {
-    s = g().toString();

[sigurdm, 2015/02/17 10:29:06]

I believer this test was wrong (the vm currently fails it for another reason).
g().toString() should not be able to throw *and* return something that can be
assigned to s.

-  } catch (e) {
-    Expect.equals("pow!", e);
-    Expect.equals("(a, b)", s);
-    print(s);
-  }
+  Iterator i = g().iterator;
+  Expect.isTrue(i.moveNext());
+  Expect.equals("a", i.current);
+  Expect.isTrue(i.moveNext());
+  Expect.equals("b", i.current);
+  Expect.throws(() => i.moveNext(), (error) => error == "pow!");
 }
 
 main() {