Print usage when no source files are passed

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 8a033a5251dc840e911932cc1e38b44baf69ab63..d9c96a329eaf65774efbfb5bec844068054eee97 100644
--- a/lib/src/compiler/command.dart
+++ b/lib/src/compiler/command.dart
@@ -67,6 +67,10 @@ 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, buildRoot, argResults.rest,
         (source) => _moduleForLibrary(moduleRoot, source));