Add validation for Flutter constraints.

test/validator/sdk_constraint_test.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 'package:pub/src/entrypoint.dart';
 import 'package:pub/src/validator.dart';
 import 'package:pub/src/validator/sdk_constraint.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 import 'utils.dart';
 
 Validator sdkConstraint(Entrypoint entrypoint) =>
   new SdkConstraintValidator(entrypoint);
 
 main() {
   group('should consider a package valid if it', () {
     integration('has no SDK constraint', () {
       d.validPackage.create();
       expectNoValidationError(sdkConstraint);
     });
 
     integration('has an SDK constraint without ^', () {
       d.dir(appPath, [
         d.libPubspec("test_pkg", "1.0.0", sdk: ">=1.8.0 <2.0.0")
       ]).create();
       expectNoValidationError(sdkConstraint);
     });
+
+    integration('has a Flutter SDK constraint with an appropriate Dart SDK '
+        'constraint', () {
+      d.dir(appPath, [
+        d.pubspec({
+          "name": "test_pkg",
+          "version": "1.0.0",
+          "environment": {
+            "sdk": ">=1.19.0 <2.0.0",
+            "flutter": "^1.2.3"
+          }
+        })
+      ]).create();
+      expectNoValidationError(sdkConstraint);
+    });
   });
 
-  test("should consider a package invalid if it has an SDK constraint with "
-      "^", () {
-    d.dir(appPath, [
-      d.libPubspec("test_pkg", "1.0.0", sdk: "^1.8.0")
-    ]).create();
-    expect(schedulePackageValidation(sdkConstraint),
-        completion(pairOf(anyElement(contains('">=1.8.0 <2.0.0"')), isEmpty)));
+  group("should consider a package invalid if it", () {
+    integration("has an SDK constraint with ^", () {
+      d.dir(appPath, [
+        d.libPubspec("test_pkg", "1.0.0", sdk: "^1.8.0")
+      ]).create();
+      expect(
+          schedulePackageValidation(sdkConstraint),
+          completion(pairOf(
+              anyElement(contains('">=1.8.0 <2.0.0"')),
+              isEmpty)));
+    });
+
+    integration("has a Flutter SDK constraint with a too-broad SDK "
+        "constraint", () {
+      d.dir(appPath, [
+        d.pubspec({
+          "name": "test_pkg",
+          "version": "1.0.0",
+          "environment": {
+            "sdk": ">=1.18.0 <1.50.0",
+            "flutter": "^1.2.3"
+          }
+        })
+      ]).create();
+      expect(
+          schedulePackageValidation(sdkConstraint),
+          completion(pairOf(
+              anyElement(contains('">=1.19.0 <1.50.0"')),
+              isEmpty)));
+    });
+
+    integration("has a Flutter SDK constraint with no SDK "
+        "constraint", () {
+      d.dir(appPath, [
+        d.pubspec({
+          "name": "test_pkg",
+          "version": "1.0.0",
+          "environment": {"flutter": "^1.2.3"}
+        })
+      ]).create();
+      expect(
+          schedulePackageValidation(sdkConstraint),
+          completion(pairOf(
+              anyElement(contains('">=1.19.0 <2.0.0"')),
+              isEmpty)));
+    });
   });
 }