Migrate pub away from throwing strings.

utils/tests/pub/command_line_config.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 command_line_config;
 
 import 'dart:io';
 import 'dart:math' as math;
 
 import 'package:pathos/path.dart' as path;
@@ skipping 135 matching lines @@
   String get location => '$library $line:$column';
 
   _StackFrame._(this.isCore, this.library, this.line, this.column, this.member);
 
   factory _StackFrame(String text) {
     var match = fileRegExp.firstMatch(text);
     var isCore = false;
 
     if (match == null) {
       match = coreRegExp.firstMatch(text);
-      if (match == null) throw "Couldn't parse stack trace line '$text'.";
+      if (match == null) {
+        throw FormatException("Couldn't parse stack trace line '$text'.");
+      }
       isCore = true;
     }
 
     var library = match[2];
     if (!isCore) {
       // Make the library path relative to the entrypoint.
       library = path.relative(library);
     }
 
     var member = match[1].replaceAll("<anonymous closure>", _lambda);
     return new _StackFrame._(isCore, library, match[3], match[4], member);
   }
 }