Fix version parsing error for new dev builds (#2186)

* Fix version parsing error for new dev builds

* Cleanup

* Correct comment to have the new version scheme as an example

Author: jcollins-g

lib/src/model/package.dart

@@ -226,9 +226,20 @@ class Package extends LibraryContainer
             case 'b':
               {
                 Version version = Version.parse(packageMeta.version);
-                return version.isPreRelease
-                    ? version.preRelease.first
-                    : 'stable';
+                String tag = 'stable';
+                if (version.isPreRelease) {
+                  // version.preRelease is a List<dynamic> with a mix of
+                  // integers and strings.  Given this, handle
+                  // 2.8.0-dev.1.0, 2.9.0-1.0.dev, and similar
+                  // variations.
+                  tag = version.preRelease.whereType<String>().first;
+                  // Who knows about non-SDK packages, but assert that SDKs
+                  // must conform to the known format.
+                  assert(
+                      packageMeta.isSdk == false || int.tryParse(tag) == null,
+                      'Got an integer as string instead of the expected "dev" tag');
+                }
+                return tag;
               }
             case 'n':
               return name;