Fix dart:html generated directory structure and unify file names between backends.

lib/html/scripts/systemnative.py

Index: lib/html/scripts/systemnative.py
diff --git a/lib/html/scripts/systemnative.py b/lib/html/scripts/systemnative.py
index 28005474e9c5d6b54fc6a04c388876dbf4806f05..ddd22cf3391dd14ec8b177256c27eeec59f2c6f3 100644
--- a/lib/html/scripts/systemnative.py
+++ b/lib/html/scripts/systemnative.py
@@ -178,18 +178,6 @@ class NativeImplementationGenerator(systembase.BaseGenerator):
   def ImplementationClassName(self):
     return self._ImplClassName(self._interface.id)
 
-  def FilePathForDartImplementation(self):
-    return os.path.join(self._system._output_dir, 'dart',
-                        '%sImplementation.dart' % self._interface.id)
-
-  def FilePathForDartFactoryProviderImplementation(self):
-    file_name = '%sFactoryProviderImplementation.dart' % self._interface.id
-    return os.path.join(self._system._output_dir, 'dart', file_name)
-
-  def FilePathForDartElementsFactoryProviderImplementation(self):
-    return os.path.join(self._system._output_dir, 'dart',
-                        '_ElementsFactoryProviderImplementation.dart')
-
   def SetImplementationEmitter(self, implementation_emitter):
     self._dart_impl_emitter = implementation_emitter