Add support for specifying SDK version constraints in pubspecs.

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 '../../pub/lock_file.dart';
@@ skipping 477 matching lines @@
     var dependencies = <PackageRef>[];
     dependencyStrings.forEach((name, constraint) {
       var parsed = parseSource(name);
       var description = parsed.first;
       var packageName = description.replaceFirst(new RegExp(r"-[^-]+$"), "");
       dependencies.add(new PackageRef(packageName, parsed.last,
           new VersionConstraint.parse(constraint), description));
     });
 
     var pubspec = new Pubspec(
-        description, new Version.parse(version), dependencies);
+        description, new Version.parse(version), dependencies,
+        new PubspecEnvironment());
     return new Package.inMemory(pubspec);
   }
 
   void addPackage(Package package) {
     _packages.putIfAbsent(package.name, () => new Map<Version, Package>());
     _packages[package.name][package.version] = package;
   }
 }
 
 /// A source used for testing that doesn't natively understand versioning,
@@ skipping 33 matching lines @@
   var match = new RegExp(r"(.*) from (.*)").firstMatch(name);
   if (match == null) return new Pair<String, Source>(name, source1);
   switch (match[2]) {
   case 'mock1': return new Pair<String, Source>(match[1], source1);
   case 'mock2': return new Pair<String, Source>(match[1], source2);
   case 'root': return new Pair<String, Source>(match[1], rootSource);
   case 'versionless':
     return new Pair<String, Source>(match[1], versionlessSource);
   }
 }