Don't allow package:-URIs as package locations when creating .packages file.

lib/packages_file.dart

 // Copyright (c) 2015, 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.
 
 library package_config.packages_file;
 
 import "package:charcode/ascii.dart";
 import "src/util.dart" show isValidPackageName;
 
 /// Parses a `.packages` file into a map from package name to base URI.
@@ skipping 62 matching lines @@
 
 /// Writes the mapping to a [StringSink].
 ///
 /// If [comment] is provided, the output will contain this comment
 /// with `# ` in front of each line.
 /// Lines are defined as ending in line feed (`'\n'`). If the final
 /// line of the comment doesn't end in a line feed, one will be added.
 ///
 /// If [baseUri] is provided, package locations will be made relative
 /// to the base URI, if possible, before writing.
+///
+/// All the keys of [packageMapping] must be valid package names,
+/// and the values must be URIs that do not have the `package:` scheme.
 void write(StringSink output, Map<String, Uri> packageMapping,
            {Uri baseUri, String comment}) {
   if (baseUri != null && !baseUri.isAbsolute) {
     throw new ArgumentError.value(baseUri, "baseUri", "Must be absolute");
   }
 
   if (comment != null) {
     var lines = comment.split('\n');
     if (lines.last.isEmpty) lines.removeLast();
     for (var commentLine in lines) {
       output.write('# ');
       output.writeln(commentLine);
     }
   } else {
     output.write("# generated by package:package_config at ");
     output.write(new DateTime.now());
     output.writeln();
   }
 
   packageMapping.forEach((String packageName, Uri uri) {
     // Validate packageName.
     if (!isValidPackageName(packageName)) {
       throw new ArgumentError('"$packageName" is not a valid package name');
     }
+    if (uri.scheme == "package") {
+      throw new ArgumentError.value(
+          "Package location must not be a package: URI", uri);
+    }
     output.write(packageName);
     output.write(':');
     // If baseUri provided, make uri relative.
     if (baseUri != null) {
       uri = _relativize(uri, baseUri);
     }
     output.write(uri);
     if (!uri.path.endsWith('/')) {
       output.write('/');
     }
@@ skipping 57 matching lines @@
   } else if (index > 0) {
     return new Uri(
         path: '../' * (base.length - index) + target.skip(index).join('/'));
   } else {
     return uri;
   }
 }
 
 // TODO: inline to uri.normalizePath() when we move to 1.11
 Uri _normalizePath(Uri uri) => new Uri().resolveUri(uri);