Migrate pub away from throwing strings.

utils/tests/pub/version_solver_test.dart

 // Copyright (c) 2012, 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 pub_update_test;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:unittest/unittest.dart';
@@ skipping 801 matching lines @@
 
   final String name;
   bool get shouldCache => true;
 
   MockSource(this.name);
 
   Future<List<Version>> getVersions(String name, String description) {
     return new Future.sync(() {
       // Make sure the solver doesn't request the same thing twice.
       if (_requestedVersions.contains(description)) {
-        throw 'Version list for $description was already requested.';
+        throw new Exception('Version list for $description was already '
+            'requested.');
       }
 
       _requestedVersions.add(description);
 
       if (!_packages.containsKey(description)){
-        throw 'MockSource does not have a package matching "$description".';
+        throw new Exception('MockSource does not have a package matching '
+            '"$description".');
       }
       return _packages[description].keys.toList();
     });
   }
 
   Future<Pubspec> describe(PackageId id) {
     return new Future.sync(() {
       // Make sure the solver doesn't request the same thing twice.
       if (_requestedPubspecs.containsKey(id.description) &&
           _requestedPubspecs[id.description].contains(id.version)) {
-        throw 'Pubspec for $id was already requested.';
+        throw new Exception('Pubspec for $id was already requested.');
       }
 
       _requestedPubspecs.putIfAbsent(id.description, () => new Set<Version>());
       _requestedPubspecs[id.description].add(id.version);
 
       return _packages[id.description][id.version].pubspec;
     });
   }
 
   Future<bool> install(PackageId id, String path) {
-    throw 'no';
+    throw new Exception('no');
   }
 
   void addPackage(String description, Package package) {
     _packages.putIfAbsent(description, () => new Map<Version, Package>());
     _packages[description][package.version] = package;
   }
 }
 
 Package mockPackage(String description, String version,
                     Map dependencyStrings) {
@@ skipping 41 matching lines @@
   };
 
   var match = new RegExp(r"(.*) from (.*)").firstMatch(description);
   if (match != null) {
     name = match[1];
     source = sourceNames[match[2]];
   }
 
   callback(isDev, name, source);
 }