Add support for Flutter SDK constraints.

lib/src/solver/version_solver.dart

Index: lib/src/solver/version_solver.dart
diff --git a/lib/src/solver/version_solver.dart b/lib/src/solver/version_solver.dart
index 83564c22c8056feb363707d76f803f84afdb3973..3c4553c26ae8ba0eaa96f0e4628376f652ca3430 100644
--- a/lib/src/solver/version_solver.dart
+++ b/lib/src/solver/version_solver.dart
@@ -79,11 +79,25 @@ class SolveResult {
   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;