anchor.diff:  
   library trace;
   
  +import 'dart:collection';
   import 'dart:uri';
   import 'dart:math' as math;
   
  
positive.code: 
  /// A stack trace, comprised of a list of stack frames.
  class Trace implements StackTrace {
    // TODO(nweiz): make this read-only once issue 8321 is fixed.
    /// The stack frames that comprise this stack trace.
    final List<Frame> frames;
  
  
negative.code: 
    /// Returns a new [Trace] comprised of [frames].
    Trace(Iterable<Frame> frames)
        : frames = new UnmodifiableListView<Frame>(frames.toList());
  
    // TODO(nweiz): Keep track of which [Frame]s are part of the partial stack
    // trace and only print them.
  