dart2js: reuse stack trace on rethrow

sdk/lib/_internal/compiler/js_lib/js_helper.dart

Index: sdk/lib/_internal/compiler/js_lib/js_helper.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/js_helper.dart b/sdk/lib/_internal/compiler/js_lib/js_helper.dart
index 02ef267082e332bf5b6d1e49159a20c8eecb49e0..f1ef96ce3eea6c47b24d678700aef78d3f404d7d 100644
--- a/sdk/lib/_internal/compiler/js_lib/js_helper.dart
+++ b/sdk/lib/_internal/compiler/js_lib/js_helper.dart
@@ -2052,7 +2052,11 @@ StackTrace getTraceFromException(exception) {
   if (exception is ExceptionAndStackTrace) {
     return exception.stackTrace;
   }
-  return new _StackTrace(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 {