Default language version (#2748)

* Added a default language version of 2.7

* Fixed tests
diff --git a/lib/src/pubspec.dart b/lib/src/pubspec.dart
index 8a9ad5c..a8c3ed2 100644
--- a/lib/src/pubspec.dart
+++ b/lib/src/pubspec.dart
@@ -447,15 +447,8 @@
       name == null && version == Version.none && dependencies.isEmpty;
 
   /// The language version implied by the sdk constraint.
-  ///
-  /// Given no or unbounded constraint we assume language version 1.0.
-  LanguageVersion get languageVersion {
-    final constraint = originalDartSdkConstraint;
-    if (constraint is VersionRange && constraint.min != null) {
-      return LanguageVersion.fromVersionRange(constraint);
-    }
-    return LanguageVersion(1, 0);
-  }
+  LanguageVersion get languageVersion =>
+      LanguageVersion.fromSdkConstraint(originalDartSdkConstraint);
 
   /// Loads the pubspec for a package located in [packageDir].
   ///