Add a stack chain class to the stack trace package.

pkg/stack_trace/lib/src/utils.dart

Index: pkg/stack_trace/lib/src/utils.dart
diff --git a/pkg/stack_trace/lib/src/utils.dart b/pkg/stack_trace/lib/src/utils.dart
index 08b3b960ec8a9599c0c57f6f44bc455d81680bf6..62a282085662d84baf6db0b38e3dc669c838f4ea 100644
--- a/pkg/stack_trace/lib/src/utils.dart
+++ b/pkg/stack_trace/lib/src/utils.dart
@@ -18,3 +18,19 @@ String padRight(String string, int length) {
   return result.toString();
 }
 
+/// Flattens nested lists inside an iterable into a single list containing only
+/// non-list elements.
+List flatten(Iterable nested) {
+  var result = [];
+  helper(list) {
+    for (var element in list) {
+      if (element is List) {
+        helper(element);
+      } else {
+        result.add(element);
+      }
+    }
+  }
+  helper(nested);
+  return result;
+}