- Allow any implementation of StackTrace to participate in async/

tests/language/custom_await_stack_trace_test.dart

+import "dart:async";
+import "package:expect/expect.dart";
+
+class Blah extends StackTrace {
+  Blah(this._trace);
+  
+  toString() {
+    return "Blah " + _trace.toString();
+  }
+  
+  var _trace;
+}
+
+foo() {
+  var x = "\nBloop\nBleep\n";
+  return new Future.error(42, new Blah(x));
+}
+
+main() async {
+  try {
+    var x = await foo();
+    Expect.fail("Should not reach here.");
+  } on int catch (e, s) {
+    Expect.equals(42, e);
+    Expect.equals("Blah \nBloop\nBleep\n", s.toString());
+    return;
+  }
+  Expect.fail("Unreachable.");
+}