Fix three strong mode analysis issues.

lib/src/version_constraint.dart

 // Copyright (c) 2014, 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_semver.src.version_constraint;
 
 import 'patterns.dart';
 import 'version.dart';
 import 'version_range.dart';
 import 'version_union.dart';
@@ skipping 96 matching lines @@
         throw new FormatException('Cannot include other constraints with '
             '"$COMPATIBLE_WITH" constraint in "$originalText".');
       }
 
       return new VersionConstraint.compatibleWith(version);
     }
 
     var compatibleWith = matchCompatibleWith();
     if (compatibleWith != null) return compatibleWith;
 
-    var constraints = [];
+    var constraints = <VersionConstraint>[];
 
     while (true) {
       skipWhitespace();
 
       if (text.isEmpty) break;
 
       var version = matchVersion();
       if (version != null) {
         constraints.add(version);
         continue;
@@ skipping 88 matching lines @@
   String toString() => '<empty>';
 }
 
 class _CompatibleWithVersionRange extends VersionRange {
   _CompatibleWithVersionRange(Version version) : super(
       min: version, includeMin: true,
       max: version.nextBreaking, includeMax: false);
 
   String toString() => '^$min';
 }