Validate that Trace.from's argument isn't null.

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:math' as math;
 
 import 'chain.dart';
@@ skipping 74 matching lines @@
       var trace = new Trace.from(nativeTrace);
       return new LazyTrace(() => new Trace(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) {
+    // Normally explicitly validating null arguments is bad Dart style, but here
+    // the natural failure will only occur when the LazyTrace is materialized,
+    // and we want to provide an error that's more local to the actual problem.
+    if (trace == null) {
+      throw new ArgumentError("Cannot create a Trace from null.");
+    }
+
     if (trace is Trace) return trace;
     if (trace is Chain) return trace.toTrace();
     return new LazyTrace(() => new Trace.parse(trace.toString()));
   }
 
   /// Parses a string representation of a stack trace.
   ///
   /// [trace] should be formatted in the same way as a Dart VM or browser stack
   /// trace.
   factory Trace.parse(String trace) {
@@ skipping 136 matching lines @@
     // Figure out the longest path so we know how much to pad.
     var longest = frames.map((frame) => frame.location.length)
         .fold(0, math.max);
 
     // Print out the stack trace nicely formatted.
     return frames.map((frame) {
       return '${padRight(frame.location, longest)}  ${frame.member}\n';
     }).join();
   }
 }