Add support for Flutter SDK constraints.

lib/src/solver/backtracking_solver.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.
 
 /// A back-tracking depth-first solver.
 ///
 /// Attempts to find the best solution for a root package's transitive
 /// dependency graph, where a "solution" is a set of concrete package versions.
 /// A valid solution will select concrete versions for every package reached
 /// from the root package's dependency graph, and each of those packages will
@@ skipping 21 matching lines @@
 /// new in-progress solution and it tries to proceed from there. It will keep
 /// doing this, traversing and then backtracking when it meets a failure until
 /// a valid solution has been found or until all possible options for all
 /// speculative choices have been exhausted.
 import 'dart:async';
 
 import 'package:pub_semver/pub_semver.dart';
 
 import '../barback.dart' as barback;
 import '../exceptions.dart';
+import '../flutter.dart' as flutter;
 import '../lock_file.dart';
 import '../log.dart' as log;
 import '../package.dart';
 import '../pubspec.dart';
 import '../sdk.dart' as sdk;
 import '../source/unknown.dart';
 import '../system_cache.dart';
 import '../utils.dart';
 import 'version_queue.dart';
 import 'version_selection.dart';
@@ skipping 588 matching lines @@
     // Indent for the previous selections.
     log.solver(prefixLines(message, prefix: '| ' * _versions.length));
   }
 
   /// Ensures that if [pubspec] has an SDK constraint, then it is compatible
   /// with the current SDK.
   ///
   /// Throws a [SolveFailure] if not.
   void _validateSdkConstraint(Pubspec pubspec) {
     if (_overrides.containsKey(pubspec.name)) return;
-    if (pubspec.environment.sdkVersion.allows(sdk.version)) return;
 
-    throw new BadSdkVersionException(pubspec.name,
-        'Package ${pubspec.name} requires SDK version '
-        '${pubspec.environment.sdkVersion} but the current SDK is '
-        '${sdk.version}.');
+    if (!pubspec.dartSdkConstraint.allows(sdk.version)) {
+      throw new BadSdkVersionException(pubspec.name,
+          'Package ${pubspec.name} requires SDK version '
+          '${pubspec.dartSdkConstraint} but the current SDK is '
+          '${sdk.version}.');
+    }
+
+    if (pubspec.flutterSdkConstraint != null) {
+      if (!flutter.isAvailable) {
+        throw new BadSdkVersionException(pubspec.name,
+            'Package ${pubspec.name} requires the Flutter SDK, which is not '
+            'available.');
+      }
+
+      if (!pubspec.flutterSdkConstraint.allows(flutter.version)) {
+        throw new BadSdkVersionException(pubspec.name,
+            'Package ${pubspec.name} requires Flutter SDK version '
+            '${pubspec.flutterSdkConstraint} but the current SDK is '
+            '${flutter.version}.');
+      }
+    }
   }
 }