Add support for Flutter SDK constraints.

lib/src/global_packages.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 'dart:async';
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
 import 'package:barback/barback.dart';
 import 'package:pub_semver/pub_semver.dart';
@@ skipping 303 matching lines @@
       entrypoint = new Entrypoint.inMemory(
           cache.load(id), lockFile, cache, isGlobal: true);
     } else {
       // For uncached sources (i.e. path), the ID just points to the real
       // directory for the package.
       entrypoint = new Entrypoint(
           (id.source as PathSource).pathFromDescription(id.description), cache,
           isGlobal: true);
     }
 
-    if (entrypoint.root.pubspec.environment.sdkVersion.allows(sdk.version)) {
-      return entrypoint;
+    if (entrypoint.root.pubspec.flutterSdkConstraint != null) {
+      dataError("${log.bold(name)} ${entrypoint.root.version} requires the "
+          "Flutter SDK, which is unsupported for global executables.");
     }
 
-    dataError("${log.bold(name)} ${entrypoint.root.version} doesn't support "
-        "Dart ${sdk.version}.");
+    if (!entrypoint.root.pubspec.dartSdkConstraint.allows(sdk.version)) {
+      dataError("${log.bold(name)} ${entrypoint.root.version} doesn't support "
+          "Dart ${sdk.version}.");
+    }
+
+    return entrypoint;
   }
 
   /// Runs [package]'s [executable] with [args].
   ///
   /// If [executable] is available in its precompiled form, that will be
   /// recompiled if the SDK has been upgraded since it was first compiled and
   /// then run. Otherwise, it will be run from source.
   ///
   /// If [checked] is true, the program is run in checked mode. If [mode] is
   /// passed, it's used as the barback mode; it defaults to
@@ skipping 451 matching lines @@
   }
 
   /// Returns the value of the property named [name] in the bin stub script
   /// [source].
   String _binStubProperty(String source, String name) {
     var pattern = new RegExp(quoteRegExp(name) + r": ([a-zA-Z0-9_-]+)");
     var match = pattern.firstMatch(source);
     return match == null ? null : match[1];
   }
 }