Remove source file check

lib/src/compiler/command.dart

Index: lib/src/compiler/command.dart
diff --git a/lib/src/compiler/command.dart b/lib/src/compiler/command.dart
index a1268c55d3097341c7e08423caa42627896b123d..545d1a4850ad19c930dca6b3ac9e67219b025c72 100644
--- a/lib/src/compiler/command.dart
+++ b/lib/src/compiler/command.dart
@@ -72,10 +72,6 @@ class CompileCommand extends Command {
       modulePath = path.basenameWithoutExtension(outPath);
     }
 
-    if (argResults.rest.isEmpty) {
-      usageException('Please pass at least one source file as an argument.');
-    }
-
     var unit = new BuildUnit(modulePath, libraryRoot, argResults.rest,
         (source) => _moduleForLibrary(moduleRoot, source));