Revert "Roll engine to version dd6f46c485192f4506035088c9065b9f5dbba9ab (#15885)" (#15980)

This caused a test failure in the integration_ui_ios devicelab test.

```
stdout: [   +3 ms] 00:02 [32m+0[0m: end-to-end test Ensure keyboard dismissal resizes the view to original size[0m
stdout: [+8129 ms] 00:11 [32m+0[0m[31m -1[0m: end-to-end test Ensure keyboard dismissal resizes the view to original size [1m[31m[E][0m[0m
stdout: [   +5 ms]   DriverError: Failed to fulfill Tap: Flutter application not responding
stdout: [        ]   Original error: TimeoutException after 0:00:07.500000: Future not completed
stdout: [        ]   Original stack trace:
stdout: [        ]   #0      FlutterDriver._sendCommand (package:flutter_driver/src/driver/driver.dart:324:18)
stdout: [        ]
stdout: [        ]   #1      FlutterDriver.tap (package:flutter_driver/src/driver/driver.dart:368:11)
stdout: [        ]
stdout: [        ]   #2      main.. (file:///[... snip ...]/flutter/dev/integration_tests/ui/test_driver/keyboard_resize_test.dart:34:20)
```

This reverts commit 067be92a.

bin/internal/engine.version

-dd6f46c485192f4506035088c9065b9f5dbba9ab
+6280adbfb1f9f63cdc6179b9b78634add1e4f2e3

packages/flutter_driver/test/flutter_driver_test.dart

@@ -29,7 +29,7 @@ void main() {
     void expectLogContains(String message) {
       expect(log.map((LogRecord r) => '$r'), anyElement(contains(message)));
     }
-    
+
     setUp(() {
       log = <LogRecord>[];
       logSub = flutterDriverLog.listen(log.add);
@@ -37,9 +37,9 @@ void main() {
       mockVM = new MockVM();
       mockIsolate = new MockIsolate();
       mockPeer = new MockPeer();
-      when(mockClient.getVM()).thenReturn(new Future<MockVM>.value(mockVM));
+      when(mockClient.getVM()).thenReturn(mockVM);
       when(mockVM.isolates).thenReturn(<VMRunnableIsolate>[mockIsolate]);
-      when(mockIsolate.loadRunnable()).thenReturn(new Future<MockIsolate>.value(mockIsolate));
+      when(mockIsolate.loadRunnable()).thenReturn(mockIsolate);
       when(mockIsolate.invokeExtension(typed(any), typed(any))).thenAnswer(
           (Invocation invocation) => makeMockResponse(<String, dynamic>{'status': 'ok'}));
       vmServiceConnectFunction = (String url) {

packages/fuchsia_remote_debug_protocol/test/fuchsia_remote_connection_test.dart

@@ -31,8 +31,7 @@ void main() {
       const String interface = 'eno1';
       // Adds some extra junk to make sure the strings will be cleaned up.
       when(mockRunner.run(typed(any)))
-      .thenReturn(new Future<List<String>>.value(
-          <String>['123\n\n\n', '456  ', '789']));
+          .thenReturn(<String>['123\n\n\n', '456  ', '789']);
       when(mockRunner.address).thenReturn(address);
       when(mockRunner.interface).thenReturn(interface);
       int port = 0;

packages/fuchsia_remote_debug_protocol/test/src/runners/ssh_command_runner_test.dart

@@ -40,8 +40,7 @@ void main() {
     setUp(() {
       mockProcessManager = new MockProcessManager();
       mockProcessResult = new MockProcessResult();
-      when(mockProcessManager.run(typed(any))).thenReturn(
-          new Future<MockProcessResult>.value(mockProcessResult));
+      when(mockProcessManager.run(typed(any))).thenReturn(mockProcessResult);
     });
 
     test('verify interface is appended to ipv6 address', () async {