Revert "fix crash when could not find package"

This reverts commit a610fa0.

example/src/examples/cow_repository.dart

@@ -132,12 +132,9 @@ class _RemoteMetadataCache {
           var c = new Completer<Set<PackageVersion>>();
 
           _versions.putIfAbsent(package, () => new Set());
-          remote.versions(package)
-            .where((v) => v != null)
-            .toList()
-            .then((versions) {
-              _versions[package].addAll(versions);
-              c.complete(_versions[package]);
+          remote.versions(package).toList().then((versions) {
+            _versions[package].addAll(versions);
+            c.complete(_versions[package]);
           });
 
           return c;

example/src/examples/http_proxy_repository.dart

@@ -27,19 +27,15 @@ class HttpProxyRepository extends PackageRepository {
         baseUrl.resolve('/api/packages/${Uri.encodeComponent(package)}');
 
     http.Response response = await client.get(versionUrl);
-    if(response.statusCode == 200) {
-      var json = convert.json.decode(response.body);
-      var versions = json['versions'] as List<dynamic>;
-      if (versions != null) {
-        for (var item in versions) {
-          var pubspec = item['pubspec'];
-          var pubspecString = convert.json.encode(pubspec);
-          yield new PackageVersion(pubspec['name'] as String,
-              pubspec['version'] as String, pubspecString);
-        }
+    var json = convert.json.decode(response.body);
+    var versions = json['versions'] as List<dynamic>;
+    if (versions != null) {
+      for (var item in versions) {
+        var pubspec = item['pubspec'];
+        var pubspecString = convert.json.encode(pubspec);
+        yield new PackageVersion(pubspec['name'] as String,
+            pubspec['version'] as String, pubspecString);
       }
-    }else{
-      yield null;
     }
   }