Make pub strong-mode clean.

lib/src/validator/sdk_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.
 
 import 'dart:async';
 
 import 'package:pub_semver/pub_semver.dart';
 
 import '../entrypoint.dart';
 import '../validator.dart';
 
 /// The range of all Dart SDK versions that don't support Flutter SDK
 /// constraints.
 final _preFlutterSupport = new VersionConstraint.parse("<1.19.0");
 
 /// A validator that validates that a package's SDK constraint doesn't use the
 /// "^" syntax.
 class SdkConstraintValidator extends Validator {
   SdkConstraintValidator(Entrypoint entrypoint)
     : super(entrypoint);
 
   Future validate() async {
     var dartConstraint = entrypoint.root.pubspec.dartSdkConstraint;
-    if (dartConstraint.toString().startsWith("^")) {
+    if (dartConstraint is VersionRange &&
+        dartConstraint.toString().startsWith("^")) {
       errors.add(
           "^ version constraints aren't allowed for SDK constraints since "
             "older versions of pub don't support them.\n"
           "Expand it manually instead:\n"
           "\n"
           "environment:\n"
           "  sdk: \">=${dartConstraint.min} <${dartConstraint.max}\"");
     }
 
     if (entrypoint.root.pubspec.flutterSdkConstraint != null &&
         dartConstraint.allowsAny(_preFlutterSupport)) {
       var newDartConstraint = dartConstraint.difference(_preFlutterSupport);
       if (newDartConstraint.isEmpty ||
           newDartConstraint ==
               VersionConstraint.any.difference(_preFlutterSupport)) {
         newDartConstraint = new VersionConstraint.parse("<2.0.0")
             .difference(_preFlutterSupport);
       }
 
       errors.add(
           "Older versions of pub don't support Flutter SDK constraints.\n"
           "Make sure your SDK constraint excludes those old versions:\n"
           "\n"
           "environment:\n"
           "  sdk: \"$newDartConstraint\"");
     }
   }
 }