Shorten help; Tell what happened if there is no explicit --out.

dart/lib/compiler/implementation/dart2js.dart

 // Copyright (c) 2012, 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.
 
 #library('dart2js');
 
 #import('dart:io');
 #import('dart:uri');
 #import('dart:utf');
 
@@ skipping 38 matching lines @@
 }
 
 void compile(List<String> argv) {
   Uri cwd = getCurrentDirectory();
   bool throwOnError = false;
   bool showWarnings = true;
   bool verbose = false;
   Uri libraryRoot = cwd;
   Uri out = cwd.resolve('out.js');
   List<String> options = new List<String>();
+  bool explicitOut = false;
 
   passThrough(String argument) => options.add(argument);
 
   List<String> arguments = <String>[];
   List<OptionHandler> handlers = <OptionHandler>[
     new OptionHandler('--throw-on-error', (_) => throwOnError = true),
     new OptionHandler('--suppress-warnings', (_) => showWarnings = false),
     new OptionHandler('--verbose', (_) => verbose = true),
     new OptionHandler('--library-root=.*', (String argument) {
       String path = nativeToUriPath(extractParameter(argument));
       if (!path.endsWith("/")) path = "$path/";
       libraryRoot = cwd.resolve(path);
     }),
     new OptionHandler('--out=.*', (String argument) {
+      explicitOut = true;
       out = cwd.resolve(nativeToUriPath(extractParameter(argument)));
     }),
     new OptionHandler('--allow-mock-compilation', passThrough),
     new OptionHandler('--no-colors', (_) => colors.enabled = false),
     new OptionHandler('--enable-checked-mode|--checked|-c',
                       (_) => passThrough('--enable_checked_mode')),
     new OptionHandler('--help', (_) => helpAndExit()),
     // The following two options must come last.
     new OptionHandler('-.*', (String argument) {
-      fail('Error: unknown option "$argument".');
+      helpAndFail('Error: Unknown option "$argument".');
     }),
     new OptionHandler('.*', (String argument) {
       arguments.add(nativeToUriPath(argument));
     })
   ];
 
   parseCommandLine(handlers, argv);
 
   if (arguments.isEmpty()) {
-    helpAndFail('Error: no file to compile.');
+    helpAndFail('Error: No Dart file specified.');
   }
   if (arguments.length > 1) {
     var extra = arguments.getRange(1, arguments.length - 1);
-    helpAndFail('Error: extra arguments: ${Strings.join(extra, " ")}');
+    helpAndFail('Error: Extra arguments: ${Strings.join(extra, " ")}');
   }
 
   Map<String, SourceFile> sourceFiles = <SourceFile>{};
   int dartBytesRead = 0;
 
   Future<String> provider(Uri uri) {
     if (uri.scheme != 'file') {
       throw new IllegalArgumentException(uri);
     }
     String source = readAll(uriPathToNative(uri.path));
@@ skipping 24 matching lines @@
     if (fatal && throwOnError) throw new AbortLeg(message);
   }
 
   Uri uri = cwd.resolve(arguments[0]);
   info('compiling $uri');
 
   // TODO(ahe): We expect the future to be complete and call value
   // directly. In effect, we don't support truly asynchronous API.
   String code = api.compile(uri, libraryRoot, provider, handler, options).value;
   if (code === null) {
-    fail('Error: compilation failed.');
+    fail('Error: Compilation failed.');
   }
   writeString(out, code);
   int jsBytesWritten = code.length;
   info('compiled $dartBytesRead bytes Dart -> $jsBytesWritten bytes JS '
        + 'in ${relativize(cwd, out)}');
+  if (!explicitOut) {
+    String input = uriPathToNative(arguments[0]);
+    String output = relativize(cwd, out);
+    print('Dart file $input compiled to JavaScript: $output');
+  }
 }
 
 class AbortLeg {
   final message;
   AbortLeg(this.message);
   toString() => 'Aborted due to --throw-on-error: $message';
 }
 
 void writeString(Uri uri, String text) {
   if (uri.scheme != 'file') {
-    fail('Error: unhandled scheme ${uri.scheme}.');
+    fail('Error: Unhandled scheme ${uri.scheme}.');
   }
   var file = new File(uriPathToNative(uri.path)).openSync(FileMode.WRITE);
   file.writeStringSync(text);
   file.closeSync();
 }
 
 String readAll(String filename) {
   var file = (new File(filename)).openSync(FileMode.READ);
   var length = file.lengthSync();
   var buffer = new List<int>(length);
@@ skipping 12 matching lines @@
   List<String> argv = ['--library-root=${options.script}$root'];
   argv.addAll(options.arguments);
   compile(argv);
 }
 
 void help() {
   // This message should be no longer than 22 lines. The default
   // terminal size normally 80x24. Two lines are used for the prompts
   // before and after running the compiler.
   print('''
-dart2js [OPTIONS...] DART-FILE
+Usage: dart2js [OPTIONS] DARTFILE
 
-A Dart to JavaScript compiler.
-
-By default, the compiled JavaScript code is saved to a file named
-out.js in the current directory.
+Compiles Dart to JavaScript.
 
 Common options:
-
   --help      Display this message.
-
-  --out=FILE  Save the output to FILE (default is out.js).
-
-  --checked   Turn on checked mode in generated JavaScript code.
-''');
+  --out=FILE  Save the output to FILE (default: out.js).
+  --checked   Turn on checked mode in generated JavaScript code.''');
 }
 
 void helpAndExit() {
   help();
   exit(0);
 }
 
 void helpAndFail(String message) {
   help();
+  print('');
   fail(message);
 }
 
 void main() {
   try {
     compilerMain(new Options());
   } catch (var exception, var trace) {
     try {
       print('Internal error: $exception');
     } catch (var ignored) {
       print('Internal error: error while printing exception');
     }
     try {
       print(trace);
     } finally {
       exit(253); // 253 is recognized as a crash by our test scripts.
     }
   }
 }