Use the version solver when choosing a version for pub global activate.

sdk/lib/_internal/pub/test/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_upgrade_test;
 
 import 'dart:async';
 
 import 'package:unittest/unittest.dart';
 
@@ skipping 1444 matching lines @@
   });
 
   var dependencyOverrides = <PackageDep>[];
   if (overrides != null) {
     overrides.forEach((spec, constraint) {
       dependencyOverrides.add(parseSpec(spec).withConstraint(
           new VersionConstraint.parse(constraint)));
     });
   }
 
-  var pubspec = new Pubspec(id.name, id.version, dependencies,
-      devDependencies, dependencyOverrides,
-      new PubspecEnvironment(sdkConstraint), []);
-  return new Package.inMemory(pubspec);
+  return new Package.inMemory(new Pubspec(id.name,
+      version: id.version,
+      dependencies: dependencies,
+      devDependencies: devDependencies,
+      dependencyOverrides: dependencyOverrides,
+      sdkConstraint: sdkConstraint));
 }
 
 /// Creates a new [PackageId] parsed from [text], which looks something like
 /// this:
 ///
 ///   foo-xyz 1.0.0 from mock
 ///
 /// The package name is "foo". A hyphenated suffix like "-xyz" here is part
 /// of the package description, but not its name, so the description here is
 /// "foo-xyz".
@@ skipping 31 matching lines @@
   }
 
   var source = "mock1";
   if (match[7] != null) {
     source = match[7];
     if (source == "root") source = null;
   }
 
   return new PackageId(name, source, parsedVersion, description);
 }