Throw an error if publish_to isn't an absolute URL (#1769)

Closes #1766

Author: nex3

lib/src/pubspec.dart

@@ -446,8 +446,12 @@ class Pubspec {
 
       // It must be "none" or a valid URL.
       if (publishTo != "none") {
-        _wrapFormatException(
-            '"publish_to" field', span, () => Uri.parse(publishTo));
+        _wrapFormatException('"publish_to" field', span, () {
+          var url = Uri.parse(publishTo);
+          if (url.scheme.isEmpty) {
+            throw new FormatException("must be an absolute URL.");
+          }
+        });
       }
     }
 

test/pubspec_test.dart

@@ -557,6 +557,11 @@ publish_to: none
         expectPubspecException('publish_to: http://bad.url:not-port',
             (pubspec) => pubspec.publishTo);
       });
+
+      test("throws on non-absolute URLs", () {
+        expectPubspecException(
+            'publish_to: pub.dartlang.org', (pubspec) => pubspec.publishTo);
+      });
     });
 
     group("executables", () {