Ensure stack trace are identical.

dart/sdk/lib/_internal/lib/js_helper.dart

Index: dart/sdk/lib/_internal/lib/js_helper.dart
diff --git a/dart/sdk/lib/_internal/lib/js_helper.dart b/dart/sdk/lib/_internal/lib/js_helper.dart
index 507e89b59ca1481ef67033c3cb4221bc38bcd52a..b92e62f02f8dbb7db7ef82a9bfb406b0aefa5561 100644
--- a/dart/sdk/lib/_internal/lib/js_helper.dart
+++ b/dart/sdk/lib/_internal/lib/js_helper.dart
@@ -1420,7 +1420,13 @@ unwrapException(ex) {
  * Called by generated code to fetch the stack trace from an
  * exception. Should never return null.
  */
-StackTrace getTraceFromException(exception) => new _StackTrace(exception);
+StackTrace getTraceFromException(exception) {
+  if (exception == null) return new _StackTrace(exception);
+  _StackTrace trace = JS('_StackTrace|Null', r'#.$cachedTrace', exception);
+  if (trace != null) return trace;
+  trace = new _StackTrace(exception);
+  return JS('_StackTrace', r'#.$cachedTrace = #', exception, trace);
+}
 
 class _StackTrace implements StackTrace {
   var _exception;