pkg/stack_trace: use List.unmodifiable when possible

lib/src/chain.dart

Index: lib/src/chain.dart
diff --git a/lib/src/chain.dart b/lib/src/chain.dart
index a27e8a68d9288ab526fb27b86a5e142d5df28bae..fdb091702d57e068301132e1831ab80b34236c6a 100644
--- a/lib/src/chain.dart
+++ b/lib/src/chain.dart
@@ -3,7 +3,6 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
-import 'dart:collection';
 import 'dart:math' as math;
 
 import 'frame.dart';
@@ -146,7 +145,7 @@ class Chain implements StackTrace {
 
   /// Returns a new [Chain] comprised of [traces].
   Chain(Iterable<Trace> traces)
-      : traces = new UnmodifiableListView<Trace>(traces.toList());
+      : traces = new List<Trace>.unmodifiable(traces);
 
   /// Returns a terser version of [this].
   ///