Support serving from multiple directories using "pub serve".

sdk/lib/_internal/pub/lib/src/command.dart

Index: sdk/lib/_internal/pub/lib/src/command.dart
diff --git a/sdk/lib/_internal/pub/lib/src/command.dart b/sdk/lib/_internal/pub/lib/src/command.dart
index d5eb6eabef02e3226ffb7e730a550f997fb03b39..92cafec76f5cdef166211d16c612d8950fc88bd4 100644
--- a/sdk/lib/_internal/pub/lib/src/command.dart
+++ b/sdk/lib/_internal/pub/lib/src/command.dart
@@ -85,7 +85,7 @@ abstract class PubCommand {
     buffer.writeln('Available ${isSubcommand ? "sub" : ""}commands:');
     for (var name in names) {
       buffer.writeln('  ${padRight(name, length)}   '
-          '${commands[name].description}');
+          '${commands[name].description.split("\n").first}');
     }
 
     return buffer.toString();