Make .packages files be more clever about defaults. (#80)

* Make `.packages` files be more clever about defaults.

The `PackageConfig` for a `.packages` file now assigns a default language version of 2.7
to all packages, and if the package location ends in `/lib/`, it assumes the package's
root directory is the parent directory of that.

* Fix test.

* Fix another test.

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 - Updated to support new rules for picking `package_config.json` over
   a specified `.packages`.
+- Deduce package root from `.packages` derived package configuration,
+  and default all such packages to language version 2.7.
 
 ## 1.9.1
 

lib/src/packages_file.dart

@@ -7,6 +7,12 @@ import "package_config_impl.dart";
 import "util.dart";
 import "errors.dart";
 
+/// The language version prior to the release of language versioning.
+///
+/// This is the default language version used by all packages from a
+/// `.packages` file.
+final LanguageVersion _languageVersion = LanguageVersion(2, 7);
+
 /// Parses a `.packages` file into a [PackageConfig].
 ///
 /// The [source] is the byte content of a `.packages` file, assumed to be
@@ -100,17 +106,24 @@ PackageConfig parse(
           "Package URI as location for package", source, separatorIndex + 1));
       continue;
     }
-    if (!packageLocation.path.endsWith('/')) {
-      packageLocation =
-          packageLocation.replace(path: packageLocation.path + "/");
+    var path = packageLocation.path;
+    if (!path.endsWith('/')) {
+      path += "/";
+      packageLocation = packageLocation.replace(path: path);
     }
     if (packageNames.contains(packageName)) {
       onError(PackageConfigFormatException(
           "Same package name occured more than once", source, start));
       continue;
     }
+    var rootUri = packageLocation;
+    if (path.endsWith("/lib/")) {
+      // Assume default Pub package layout. Include package itself in root.
+      rootUri =
+          packageLocation.replace(path: path.substring(0, path.length - 4));
+    }
     var package = SimplePackage.validate(
-        packageName, packageLocation, packageLocation, null, null, (error) {
+        packageName, rootUri, packageLocation, _languageVersion, null, (error) {
       if (error is ArgumentError) {
         onError(PackageConfigFormatException(error.message, source));
       } else {

test/parse_test.dart

@@ -34,9 +34,9 @@ void main() {
 
       var foo = result["foo"];
       expect(foo, isNotNull);
-      expect(foo.root, Uri.parse("file:///foo/lib/"));
+      expect(foo.root, Uri.parse("file:///foo/"));
       expect(foo.packageUriRoot, Uri.parse("file:///foo/lib/"));
-      expect(foo.languageVersion, null);
+      expect(foo.languageVersion, LanguageVersion(2, 7));
     });
 
     test("valid empty", () {