Use the chrome.input.ime API to implement a custom IME for Chrome OS. This allows your extension to handle keystrokes, set the composition, and manage the candidate window.

class ChromeInputIme extends ChromeApi {
 static final JsObject _input_ime = chrome['input']['ime'];

 ChromeInputIme._();

 bool get available => _input_ime != null;

 /**
  * Set the current composition. If this extension does not own the active IME,
  * this fails.
  */
 Future<bool> setComposition(InputImeSetCompositionParams parameters) {
   if (_input_ime == null) _throwNotAvailable();

   var completer = new ChromeCompleter<bool>.oneArg();
   _input_ime.callMethod('setComposition', [jsify(parameters), completer.callback]);
   return completer.future;
 }

 /**
  * Clear the current composition. If this extension does not own the active
  * IME, this fails.
  */
 Future<bool> clearComposition(InputImeClearCompositionParams parameters) {
   if (_input_ime == null) _throwNotAvailable();

   var completer = new ChromeCompleter<bool>.oneArg();
   _input_ime.callMethod('clearComposition', [jsify(parameters), completer.callback]);
   return completer.future;
 }

 /**
  * Commits the provided text to the current input.
  */
 Future<bool> commitText(InputImeCommitTextParams parameters) {
   if (_input_ime == null) _throwNotAvailable();

   var completer = new ChromeCompleter<bool>.oneArg();
   _input_ime.callMethod('commitText', [jsify(parameters), completer.callback]);
   return completer.future;
 }

 /**
  * Sets the properties of the candidate window. This fails if the extension
  * doesn’t own the active IME
  */
 Future<bool> setCandidateWindowProperties(InputImeSetCandidateWindowPropertiesParams parameters) {
   if (_input_ime == null) _throwNotAvailable();

   var completer = new ChromeCompleter<bool>.oneArg();
   _input_ime.callMethod('setCandidateWindowProperties', [jsify(parameters), completer.callback]);
   return completer.future;
 }

 /**
  * Sets the current candidate list. This fails if this extension doesn’t own
  * the active IME
  */
 Future<bool> setCandidates(InputImeSetCandidatesParams parameters) {
   if (_input_ime == null) _throwNotAvailable();

   var completer = new ChromeCompleter<bool>.oneArg();
   _input_ime.callMethod('setCandidates', [jsify(parameters), completer.callback]);
   return completer.future;
 }

 /**
  * Set the position of the cursor in the candidate window. This is a no-op if
  * this extension does not own the active IME.
  */
 Future<bool> setCursorPosition(InputImeSetCursorPositionParams parameters) {
   if (_input_ime == null) _throwNotAvailable();

   var completer = new ChromeCompleter<bool>.oneArg();
   _input_ime.callMethod('setCursorPosition', [jsify(parameters), completer.callback]);
   return completer.future;
 }

 /**
  * Adds the provided menu items to the language menu when this IME is active.
  */
 Future setMenuItems(InputImeSetMenuItemsParams parameters) {
   if (_input_ime == null) _throwNotAvailable();

   var completer = new ChromeCompleter.noArgs();
   _input_ime.callMethod('setMenuItems', [jsify(parameters), completer.callback]);
   return completer.future;
 }

 /**
  * Updates the state of the MenuItems specified
  */
 Future updateMenuItems(InputImeUpdateMenuItemsParams parameters) {
   if (_input_ime == null) _throwNotAvailable();

   var completer = new ChromeCompleter.noArgs();
   _input_ime.callMethod('updateMenuItems', [jsify(parameters), completer.callback]);
   return completer.future;
 }

 /**
  * Deletes the text around the caret.
  */
 Future deleteSurroundingText(InputImeDeleteSurroundingTextParams parameters) {
   if (_input_ime == null) _throwNotAvailable();

   var completer = new ChromeCompleter.noArgs();
   _input_ime.callMethod('deleteSurroundingText', [jsify(parameters), completer.callback]);
   return completer.future;
 }

 /**
  * Indicates that the key event received by onKeyEvent is handled.  This
  * should only be called if the onKeyEvent listener is asynchronous.
  * 
  * [requestId] Request id of the event that was handled.  This should come
  * from keyEvent.requestId
  * 
  * [response] True if the keystroke was handled, false if not
  */
 void keyEventHandled(String requestId, bool response) {
   if (_input_ime == null) _throwNotAvailable();

   _input_ime.callMethod('keyEventHandled', [requestId, response]);
 }

 /**
  * This event is sent when an IME is activated. It signals that the IME will
  * be receiving onKeyPress events.
  */
 Stream<String> get onActivate => _onActivate.stream;

 final ChromeStreamController<String> _onActivate =
     new ChromeStreamController<String>.oneArg(_input_ime, 'onActivate', selfConverter);

 /**
  * This event is sent when an IME is deactivated. It signals that the IME will
  * no longer be receiving onKeyPress events.
  */
 Stream<String> get onDeactivated => _onDeactivated.stream;

 final ChromeStreamController<String> _onDeactivated =
     new ChromeStreamController<String>.oneArg(_input_ime, 'onDeactivated', selfConverter);

 /**
  * This event is sent when focus enters a text box. It is sent to all
  * extensions that are listening to this event, and enabled by the user.
  */
 Stream<InputContext> get onFocus => _onFocus.stream;

 final ChromeStreamController<InputContext> _onFocus =
     new ChromeStreamController<InputContext>.oneArg(_input_ime, 'onFocus', _createInputContext);

 /**
  * This event is sent when focus leaves a text box. It is sent to all
  * extensions that are listening to this event, and enabled by the user.
  */
 Stream<int> get onBlur => _onBlur.stream;

 final ChromeStreamController<int> _onBlur =
     new ChromeStreamController<int>.oneArg(_input_ime, 'onBlur', selfConverter);

 /**
  * This event is sent when the properties of the current InputContext change,
  * such as the the type. It is sent to all extensions that are listening to
  * this event, and enabled by the user.
  */
 Stream<InputContext> get onInputContextUpdate => _onInputContextUpdate.stream;

 final ChromeStreamController<InputContext> _onInputContextUpdate =
     new ChromeStreamController<InputContext>.oneArg(_input_ime, 'onInputContextUpdate', _createInputContext);

 /**
  * This event is sent if this extension owns the active IME.
  */
 Stream<OnKeyEventEvent> get onKeyEvent => _onKeyEvent.stream;

 final ChromeStreamController<OnKeyEventEvent> _onKeyEvent =
     new ChromeStreamController<OnKeyEventEvent>.twoArgs(_input_ime, 'onKeyEvent', _createOnKeyEventEvent);

 /**
  * This event is sent if this extension owns the active IME.
  */
 Stream<OnCandidateClickedEvent> get onCandidateClicked => _onCandidateClicked.stream;

 final ChromeStreamController<OnCandidateClickedEvent> _onCandidateClicked =
     new ChromeStreamController<OnCandidateClickedEvent>.threeArgs(_input_ime, 'onCandidateClicked', _createOnCandidateClickedEvent);

 /**
  * Called when the user selects a menu item
  */
 Stream<OnMenuItemActivatedEvent> get onMenuItemActivated => _onMenuItemActivated.stream;

 final ChromeStreamController<OnMenuItemActivatedEvent> _onMenuItemActivated =
     new ChromeStreamController<OnMenuItemActivatedEvent>.twoArgs(_input_ime, 'onMenuItemActivated', _createOnMenuItemActivatedEvent);

 /**
  * Called when the editable string around caret is changed or when the caret
  * position is moved. The text length is limited to 100 characters for each
  * back and forth direction.
  */
 Stream<OnSurroundingTextChangedEvent> get onSurroundingTextChanged => _onSurroundingTextChanged.stream;

 final ChromeStreamController<OnSurroundingTextChangedEvent> _onSurroundingTextChanged =
     new ChromeStreamController<OnSurroundingTextChangedEvent>.twoArgs(_input_ime, 'onSurroundingTextChanged', _createOnSurroundingTextChangedEvent);

 /**
  * This event is sent when chrome terminates ongoing text input session.
  */
 Stream<String> get onReset => _onReset.stream;

 final ChromeStreamController<String> _onReset =
     new ChromeStreamController<String>.oneArg(_input_ime, 'onReset', selfConverter);

 void _throwNotAvailable() {
   throw new UnsupportedError("'chrome.input.ime' is not available");
 }
}