Migrate pub away from throwing strings.

utils/tests/pub/test_pub.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.
 
 /// Test infrastructure for testing pub. Unlike typical unit tests, most pub
 /// tests are integration tests that stage some stuff on the file system, run
 /// pub, and then validate the results. This library provides an API to build
 /// tests like that.
 library test_pub;
 
@@ skipping 453 matching lines @@
       case "git":
         source = new GitSource();
         break;
       case "hosted":
         source = new HostedSource();
         break;
       case "path":
         source = new PathSource();
         break;
       default:
-        throw 'Unknown source "$sourceName"';
+        throw new Exception('Unknown source "$sourceName"');
       }
 
       result[_packageName(sourceName, dependency[sourceName])] = dependency;
     }
     return result;
   });
 }
 
 /// Return the name for the package described by [description] and from
 /// [sourceName].
@@ skipping 125 matching lines @@
   bool matches(item, MatchState matchState) {
     if (item is! Pair) return false;
     return _firstMatcher.matches(item.first, matchState) &&
         _lastMatcher.matches(item.last, matchState);
   }
 
   Description describe(Description description) {
     description.addAll("(", ", ", ")", [_firstMatcher, _lastMatcher]);
   }
 }