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

lib/packages_file.dart

Index: lib/packages_file.dart
diff --git a/lib/packages_file.dart b/lib/packages_file.dart
index 73e6061c9f6faf12a74e54123b13aa924cecfc8c..25d2d6883bb29ad774192cf37bf42e9443dd64ea 100644
--- a/lib/packages_file.dart
+++ b/lib/packages_file.dart
@@ -80,6 +80,9 @@ Map<String, Uri> parse(List<int> source, Uri baseLocation) {
 ///
 /// 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) {
@@ -104,6 +107,10 @@ void write(StringSink output, Map<String, Uri> packageMapping,
     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.