Make the oauth2 lib handle OAuth2 401 errors.

pkg/oauth2/lib/src/utils.dart

Index: pkg/oauth2/lib/src/utils.dart
diff --git a/pkg/oauth2/lib/src/utils.dart b/pkg/oauth2/lib/src/utils.dart
index 583a5bc4438ecea546e41cb8b833b235448676de..59428b3c6fa906927c3f067df2f736c619467b9a 100644
--- a/pkg/oauth2/lib/src/utils.dart
+++ b/pkg/oauth2/lib/src/utils.dart
@@ -65,6 +65,52 @@ List<String> split1(String toSplit, String pattern) {
       toSplit.substring(index + pattern.length)];
 }
 
+/// A WWW-Authenticate header value, parsed as per [RFC 2617][].
+///
+/// [RFC 2617]: http://tools.ietf.org/html/rfc2617
+class AuthenticateHeader {
+  final String scheme;
+  final Map<String, String> parameters;
+
+  AuthenticateHeader(this.scheme, this.parameters);
+
+  /// Parses a header string. Throws a [FormatException] if the header is
+  /// invalid.
+  factory AuthenticateHeader.parse(String header) {
+    var split = split1(header, ' ');
+    if (split.length == 0) {
+      throw new FormatException('Invalid WWW-Authenticate header: "$header"');
+    } else if (split.length == 1 || split[1].trim().isEmpty) {
+      return new AuthenticateHeader(split[0].toLowerCase(), {});
+    }
+    var scheme = split[0].toLowerCase();
+    var paramString = split[1];
+
+    // From http://www.w3.org/Protocols/rfc2616/rfc2616-sec2.html.
+    var tokenChar = r'[^\0-\x1F()<>@,;:\\"/\[\]?={} \t\x7F]';
+    var quotedStringChar = r'(?:[^\0-\x1F\x7F"]|\\.)';
+    var regexp = new RegExp('^ *($tokenChar+)="($quotedStringChar*)" *(, *)?');
+
+    var parameters = {};
+    var match;
+    do {
+      match = regexp.firstMatch(paramString);
+      if (match == null) {
+        throw new FormatException('Invalid WWW-Authenticate header: "$header"');
+      }
+
+      paramString = paramString.substring(match.end);
+      parameters[match.group(1).toLowerCase()] = match.group(2);
+    } while (match.group(3) != null);
+
+    if (!paramString.trim().isEmpty) {
+      throw new FormatException('Invalid WWW-Authenticate header: "$header"');
+    }
+
+    return new AuthenticateHeader(scheme, parameters);
+  }
+}
+
 /// Returns a [Future] that asynchronously completes to `null`.
 Future get async {
   var completer = new Completer();