Set --fail-on-severe always for webdev build.

Closes #44

Author: alorenzen

webdev/lib/src/command/build_command.dart

@@ -26,6 +26,6 @@ class BuildCommand extends CommandBase {
           '"${argResults.rest.join(' ')}".',
           argParser.usage);
     }
-    return runCore('build');
+    return runCore('build', extraArgs: ['--fail-on-severe']);
   }
 }

webdev/lib/src/command/command_base.dart

@@ -68,14 +68,15 @@ abstract class CommandBase extends Command<int> {
     return arguments;
   }
 
-  Future<int> runCore(String command) async {
+  Future<int> runCore(String command,
+      {List<String> extraArgs = const []}) async {
     await checkPubspecLock(
         requireBuildWebCompilers:
             argResults[_requireBuildWebCompilers] as bool);
 
     var buildRunnerScript = await _buildRunnerScript();
 
-    final arguments = [command]..addAll(getArgs());
+    final arguments = [command]..addAll(extraArgs)..addAll(getArgs());
 
     var exitCode = 0;