Revert "Dart SDK roll for 2018/09/06 (#6189)" (flutter/engine#6192)

This reverts commit 2445a6e8e9.

DEPS

@@ -31,7 +31,7 @@ vars = {
   # Dart is: https://github.com/dart-lang/sdk/blob/master/DEPS.
   # You can use //tools/dart/create_updated_flutter_deps.py to produce
   # updated revision list of existing dependencies.
-  'dart_revision': 'c7ab56e1b5ab9bcf6f20057ac9135dede7d40395',
+  'dart_revision': '760a9690c22ec3f3d163173737f9949f97e6e02a',
 
   'dart_args_tag': '1.4.4',
   'dart_async_tag': '2.0.8',

engine/src/flutter/frontend_server/lib/server.dart

@@ -39,11 +39,6 @@ class _FlutterFrontendCompiler implements frontend.CompilerInterface{
     _compiler.acceptLastDelta();
   }
 
-  @override
-  Future<Null> rejectLastDelta() async {
-    return _compiler.rejectLastDelta();
-  }
-
   @override
   void invalidate(Uri uri) {
     _compiler.invalidate(uri);