Bubble up exceptions throw async/await.

runtime/lib/core_patch.dart

Index: runtime/lib/core_patch.dart
diff --git a/runtime/lib/core_patch.dart b/runtime/lib/core_patch.dart
index 549169878f6b466aac652ba9515e297382829a32..b8a85245ed8083ad253670e48945479b29eadaee 100644
--- a/runtime/lib/core_patch.dart
+++ b/runtime/lib/core_patch.dart
@@ -4,3 +4,9 @@
 
 import "dart:math";
 import "dart:typed_data";
+
+// We need to pass the exception object as second parameter to the continuation.
+// See vm/ast_transformer.cc for usage.
+void  _asyncCatchHelper(catchFunction, continuation) {
+  catchFunction((e) => continuation(null, e));
+}