Be lazier when capturing stack traces.

pkg/stack_trace/lib/src/trace.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library trace;
 
 import 'dart:collection';
 import 'dart:uri';
 import 'dart:math' as math;
 
 import 'frame.dart';
+import 'lazy_trace.dart';
 
 final _patchRegExp = new RegExp(r"-patch$");
 
 /// A stack trace, comprised of a list of stack frames.
 class Trace implements StackTrace {
   /// The stack frames that comprise this stack trace.
   final List<Frame> frames;
 
   /// Returns a human-readable representation of [stackTrace]. If [terse] is
   /// set, this folds together multiple stack frames from the Dart core
@@ skipping 13 matching lines @@
   factory Trace.current([int level=0]) {
     if (level < 0) {
       throw new ArgumentError("Argument [level] must be greater than or equal "
           "to 0.");
     }
 
     try {
       throw '';
     } catch (_, nativeTrace) {
       var trace = new Trace.from(nativeTrace);
-      return new Trace(trace.frames.skip(level + 1));
+      return new LazyTrace(() => trace.frames.skip(level + 1));
     }
   }
 
   /// Returns a new stack trace containing the same data as [trace].
   ///
   /// If [trace] is a native [StackTrace], its data will be parsed out; if it's
   /// a [Trace], it will be returned as-is.
   factory Trace.from(StackTrace trace) {
     if (trace is Trace) return trace;
-    return new Trace.parse(trace.toString());
+    return new LazyTrace(() => Trace.parse(trace.toString()));
   }
 
   /// Parses a string representation of a stack trace.
   ///
   /// [trace] should be formatted in the same way as native stack traces.
   Trace.parse(String trace)
       : this(trace.trim().split("\n").map((line) => new Frame.parse(line)));
 
   /// Returns a new [Trace] comprised of [frames].
   Trace(Iterable<Frame> frames)
@@ skipping 66 matching lines @@
   if (string.length >= length) return string;
 
   var result = new StringBuffer();
   result.write(string);
   for (var i = 0; i < length - string.length; i++) {
     result.write(' ');
   }
 
   return result.toString();
 }