Add package implementation.

lib/src/utils.dart

Index: lib/src/utils.dart
diff --git a/lib/src/utils.dart b/lib/src/utils.dart
new file mode 100644
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+++ b/lib/src/utils.dart
@@ -0,0 +1,73 @@
+// Copyright (c) 2016, 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.
+
+// TODO(nweiz): Avoid importing dart:io directly when cross-platform libraries
+// exist.
+import 'dart:io';
+import 'dart:isolate';
+import 'dart:async';
+import 'dart:convert';
+
+import 'package:http/http.dart' as http;
+import 'package:package_config/packages_file.dart' as packages_file;
+
+/// Loads the configuration map from [uri].
+///
+/// This supports `http`, `file`, `data`, and `package` URIs. If it's passed and
+/// an HTTP request is needed, [client] is used to make that request.
+Future<Map<String, Uri>> loadConfigMap(Uri uri, {http.Client client}) async {
+  var resolved = Uri.base.resolveUri(uri);
+
+  var text;
+  if (resolved.scheme == 'http') {
+    text = await (client == null
+        ? http.read(resolved)
+        : client.read(resolved));
+  } else if (resolved.scheme == 'file') {
+    var path = resolved.toFilePath(windows: Platform.isWindows);
+    text = await new File(path).readAsString();
+  } else if (resolved.scheme == 'data') {
+    text = resolved.data.contentAsString();
+  } else if (resolved.scheme == 'package') {
+    return loadConfigMap(await Isolate.resolvePackageUri(uri),
+        client: client);
+  } else {
+    throw new UnsupportedError(
+        'PackageInfo.loadConfig doesn\'t support URI scheme "${uri.scheme}:".');
+  }
+
+  return packages_file.parse(UTF8.encode(text), resolved);
+}
+
+/// Converts [uri] to a [Uri] and verifies that it's a valid `package:` URI.
+///
+/// Throws an [ArgumentError] if [uri] isn't a [String] or a [Uri]. [name] is
+/// used as the argument name in that error.
+///
+/// Throws a [FormatException] if [uri] isn't a `package:` URI or doesn't have
+/// at least one path segment.
+Uri asPackageUri(uri, String name) {
+  uri = asUri(uri, name);
+
+  if (uri.scheme != 'package') {
+    throw new FormatException("Can only resolve a package: URI.",
+        uri.toString(), 0);
+  } else if (uri.pathSegments.isEmpty) {
+    throw new FormatException("Expected package name.",
+        uri.toString(), "package:".length);
+  }
+
+  return uri;
+}
+
+/// Converts [uri] to a [Uri].
+///
+/// Throws an [ArgumentError] if [uri] isn't a [String] or a [Uri]. [name] is
+/// used as the argument name in that error.
+Uri asUri(uri, String name) {
+  if (uri is Uri) return uri;
+  if (uri is String) return Uri.parse(uri);
+
+  throw new ArgumentError.value(uri, name, "Must be a String or a Uri.");
+}