dart2js: Add a header to the output of the startup emitter.

pkg/compiler/lib/src/js_emitter/headers.dart

+// Copyright (c) 2015, 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.js_emitter.headers;
+import '../apiimpl.dart' show Compiler;
+
+String generatedBy(Compiler compiler, {String flavor: ""}) {
+  String suffix = '';
+  if (compiler.hasBuildId) suffix = ' version: ${compiler.buildId}';
+  if (flavor != "") flavor = " ($flavor)";
+  return '// Generated by dart2js$flavor, '
+      'the Dart to JavaScript compiler$suffix.';
+}
+
+const String HOOKS_API_USAGE = """
+// The code supports the following hooks:
+// dartPrint(message):
+//    if this function is defined it is called instead of the Dart [print]
+//    method.
+//
+// dartMainRunner(main, args):
+//    if this function is defined, the Dart [main] method will not be invoked
+//    directly. Instead, a closure that will invoke [main], and its arguments
+//    [args] is passed to [dartMainRunner].
+//
+// dartDeferredLibraryLoader(uri, successCallback, errorCallback):
+//    if this function is defined, it will be called when a deferered library
+//    is loaded. It should load and eval the javascript of `uri`, and call
+//    successCallback. If it fails to do so, it should call errorCallback with
+//    an error.
+""";