Add support for Flutter SDK constraints.

lib/src/solver/version_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.
 
 import 'dart:async';
 import "dart:convert";
 
 import 'package:pub_semver/pub_semver.dart';
 import 'package:stack_trace/stack_trace.dart';
 
@@ skipping 61 matching lines @@
   ///
   /// In other words, one more than the number of times it had to backtrack
   /// because it found an invalid solution.
   final int attemptedSolutions;
 
   /// The [LockFile] representing the packages selected by this version
   /// resolution.
   LockFile get lockFile {
     // Don't factor in overridden dependencies' SDK constraints, because we'll
     // accept those packages even if their constraints don't match.
-    var sdkConstraint = new VersionConstraint.intersection(pubspecs.values
+    var nonOverrides = pubspecs.values
         .where((pubspec) =>
             !_root.dependencyOverrides.any((dep) => dep.name == pubspec.name))
-        .map((pubspec) => pubspec.environment.sdkVersion));
-    return new LockFile(packages, sdkConstraint: sdkConstraint);
+        .toList();
+
+    var dartMerged = new VersionConstraint.intersection(nonOverrides
+        .map((pubspec) => pubspec.dartSdkConstraint));
+
+    var flutterConstraints = nonOverrides
+        .map((pubspec) => pubspec.flutterSdkConstraint)
+        .where((constraint) => constraint != null)
+        .toList();
+    var flutterMerged = flutterConstraints.isEmpty
+        ? null
+        : new VersionConstraint.intersection(flutterConstraints);
+
+    return new LockFile(packages,
+        dartSdkConstraint: dartMerged,
+        flutterSdkConstraint: flutterMerged);
   }
 
   final SourceRegistry _sources;
   final Package _root;
   final LockFile _previousLockFile;
 
   /// Returns the names of all packages that were changed.
   ///
   /// This includes packages that were added or removed.
   Set<String> get changedPackages {
@@ skipping 339 matching lines @@
 
   DependencyNotFoundException(String package, this._innerException,
       Iterable<Dependency> dependencies)
       : super(package, dependencies);
 
   /// The failure isn't because of the version of description of the package,
   /// it's the package itself that can't be found, so just show the name and no
   /// descriptive details.
   String _describeDependency(PackageDep dep) => "";
 }