Add support for Flutter SDK constraints.

lib/src/entrypoint.dart

Index: lib/src/entrypoint.dart
diff --git a/lib/src/entrypoint.dart b/lib/src/entrypoint.dart
index 360a6ca5d0c44eba2cf382b05ae2fb5f640d4f45..cc58a3e3ed214e8278c9390c7a4091a890eb2c0c 100644
--- a/lib/src/entrypoint.dart
+++ b/lib/src/entrypoint.dart
@@ -12,6 +12,7 @@ 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;
@@ -24,8 +25,26 @@ 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.
 ///
@@ -489,14 +508,29 @@ class Entrypoint {
       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