Print usage when no source files are passed

lib/src/compiler/command.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:convert' show JSON;
 import 'dart:io';
 import 'package:args/command_runner.dart';
 import 'package:analyzer/src/generated/source.dart' show Source;
 import 'package:analyzer/src/summary/package_bundle_reader.dart'
     show InSummarySource;
@@ skipping 49 matching lines @@
         usageException('Output file $outPath must be within the module root '
             'directory $moduleRoot');
       }
       modulePath =
           path.withoutExtension(path.relative(outPath, from: moduleRoot));
     } else {
       moduleRoot = path.dirname(outPath);
       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));
 
     JSModuleFile module = compiler.compile(unit, compilerOptions);
     module.errors.forEach(messageHandler);
 
     if (!module.isValid) throw new CompileErrorException();
 
     // Write JS file, as well as source map and summary (if requested).
     new File(outPath).writeAsStringSync(module.code);
@@ skipping 25 matching lines @@
         'Imported file "${source.uri}" was not found as a summary or source '
         'file. Please pass in either the summary or the source file '
         'for this import.');
   }
 }
 
 /// Thrown when the input source code has errors.
 class CompileErrorException implements Exception {
   toString() => '\nPlease fix all errors before compiling (warnings are okay).';
 }