Add support for Flutter SDK constraints.

lib/src/entrypoint.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:io';
 
 import 'package:barback/barback.dart';
 import 'package:package_config/packages_file.dart' as packages_file;
 import 'package:path/path.dart' as p;
 import 'package:pub_semver/pub_semver.dart';
 
 import 'barback/asset_environment.dart';
 import 'exceptions.dart';
+import 'flutter.dart' as flutter;
 import 'io.dart';
 import 'lock_file.dart';
 import 'log.dart' as log;
 import 'package.dart';
 import 'package_graph.dart';
 import 'sdk.dart' as sdk;
 import 'solver/version_solver.dart';
 import 'source/cached.dart';
 import 'source/unknown.dart';
 import 'system_cache.dart';
 import 'utils.dart';
 
-/// A RegExp to match the SDK constraint in a lock file.
-final _sdkConstraint = new RegExp(r'^sdk: "?([^"]*)"?$', multiLine: true);
+/// A RegExp to match the Dart SDK constraint in a lock file.
+///
+/// This matches both the old-style constraint:
+///
+/// ```yaml
+/// sdk: ">=1.2.3 <2.0.0"
+/// ```
+///
+/// and the new-style constraint:
+///
+/// ```yaml
+/// sdks:
+///   dart: ">=1.2.3 <2.0.0"
+/// ```
+final _dartSdkConstraint =
+    new RegExp(r'^(  dart|sdk): "?([^"]*)"?$', multiLine: true);
+
+/// A RegExp to match the Flutter SDK constraint in a lock file.
+final _flutterSdkConstraint =
+    new RegExp(r'^  flutter: "?([^"]*)"?$', multiLine: true);
 
 /// The context surrounding the root package pub is operating on.
 ///
 /// Pub operates over a directed graph of dependencies that starts at a root
 /// "entrypoint" package. This is typically the package where the current
 /// working directory is located. An entrypoint knows the [root] package it is
 /// associated with and is responsible for managing the "packages" directory
 /// for it.
 ///
 /// That directory contains symlinks to all packages used by an app. These links
@@ skipping 443 matching lines @@
       if (_isPackagesFileUpToDate()) {
         touch(packagesFile);
       } else {
         dataError('The pubspec.lock file has changed since the .packages file '
             'was generated, please run "pub get" again.');
       }
     } else if (touchedLockFile) {
       touch(packagesFile);
     }
 
-    var sdkConstraint = _sdkConstraint.firstMatch(lockFileText);
-    if (sdkConstraint != null) {
-      var parsedConstraint = new VersionConstraint.parse(sdkConstraint[1]);
+    var dartSdkConstraint = _dartSdkConstraint.firstMatch(lockFileText);
+    if (dartSdkConstraint != null) {
+      var parsedConstraint = new VersionConstraint.parse(dartSdkConstraint[2]);
       if (!parsedConstraint.allows(sdk.version)) {
         dataError("Dart ${sdk.version} is incompatible with your dependencies' "
             "SDK constraints. Please run \"pub get\" again.");
       }
     }
+
+    // Don't complain if there's a Flutter constraint but Flutter is
+    // unavailable. Flutter being unavailable just means that we aren't running
+    // from within the `flutter` executable, and we want users to be able to
+    // `pub run` non-Flutter tools even in a Flutter app.
+    var flutterSdkConstraint = _flutterSdkConstraint.firstMatch(lockFileText);
+    if (flutterSdkConstraint != null && flutter.isAvailable) {
+      var parsedConstraint = new VersionConstraint.parse(
+          flutterSdkConstraint[1]);
+
+      if (!parsedConstraint.allows(flutter.version)) {
+        dataError("Flutter ${flutter.version} is incompatible with your "
+            "dependencies' SDK constraints. Please run \"pub get\" again.");
+      }
+    }
   }
 
   /// Determines whether or not the lockfile is out of date with respect to the
   /// pubspec.
   ///
   /// If any mutable pubspec contains dependencies that are not in the lockfile
   /// or that don't match what's in there, this will throw a [DataError]
   /// describing the issue.
   void _assertLockFileUpToDate() {
     if (!root.immediateDependencies.every(_isDependencyUpToDate)) {
@@ skipping 163 matching lines @@
   /// If [packageSymlinks] is true, creates a symlink to the "packages"
   /// directory in [dir].
   ///
   /// Otherwise, deletes a "packages" directories in [dir] if one exists.
   void _linkOrDeleteSecondaryPackageDir(String dir) {
     var symlink = p.join(dir, 'packages');
     if (entryExists(symlink)) deleteEntry(symlink);
     if (_packageSymlinks) createSymlink(packagesDir, symlink, relative: true);
   }
 }