Revert "Roll Engine from 20caf54 to d2ad441 (27 revisions) (flutter#71498)"

This appears to be triggering an null dereference by calling
popSystemNavigator on a null platform plugin delegate.

boolean io.flutter.plugin.platform.PlatformPlugin$PlatformPluginDelegate.popSystemNavigator()

Relevant logs:
```
2020-12-01T14:34:34.410242: stderr: [   +6 ms] VMServiceFlutterDriver: >>> {command: get_text, finderType: ByValueKey, keyValueString: status, keyValueType: String}
2020-12-01T14:34:34.490211: stderr: [  +79 ms] VMServiceFlutterDriver: <<< {isError: false, response: {text: ok}, type: _extensionType, method: ext.flutter.driver}
2020-12-01T14:34:34.493011: stderr: [   +2 ms] VMServiceFlutterDriver: >>> {command: tap, finderType: ByValueKey, keyValueString: step, keyValueType: String}
2020-12-01T14:34:34.962001: stdout: [ +468 ms] E/MethodChannel#flutter/platform( 9732): Failed to handle method call
2020-12-01T14:34:34.962222: stdout: [        ] E/MethodChannel#flutter/platform( 9732): java.lang.NullPointerException: Attempt to invoke interface method 'boolean io.flutter.plugin.platform.PlatformPlugin$PlatformPluginDelegate.popSystemNavigator()' on a null object reference
2020-12-01T14:34:34.962356: stdout: [        ] E/MethodChannel#flutter/platform( 9732):   at io.flutter.plugin.platform.PlatformPlugin.popSystemNavigator(PlatformPlugin.java:313)
2020-12-01T14:34:34.962499: stdout: [        ] E/MethodChannel#flutter/platform( 9732):   at io.flutter.plugin.platform.PlatformPlugin.access$600(PlatformPlugin.java:28)
2020-12-01T14:34:34.962735: stdout: [        ] E/MethodChannel#flutter/platform( 9732):   at io.flutter.plugin.platform.PlatformPlugin$1.popSystemNavigator(PlatformPlugin.java:101)
2020-12-01T14:34:34.963047: stdout: [        ] E/MethodChannel#flutter/platform( 9732):   at io.flutter.embedding.engine.systemchannels.PlatformChannel$1.onMethodCall(PlatformChannel.java:123)
2020-12-01T14:34:34.963284: stdout: [        ] E/MethodChannel#flutter/platform( 9732):   at io.flutter.plugin.common.MethodChannel$IncomingMethodCallHandler.onMessage(MethodChannel.java:233)
2020-12-01T14:34:34.963592: stdout: [        ] E/MethodChannel#flutter/platform( 9732):   at io.flutter.embedding.engine.dart.DartMessenger.handleMessageFromDart(DartMessenger.java:85)
2020-12-01T14:34:34.963910: stdout: [        ] E/MethodChannel#flutter/platform( 9732):   at io.flutter.embedding.engine.FlutterJNI.handlePlatformMessage(FlutterJNI.java:738)
2020-12-01T14:34:34.964227: stdout: [        ] E/MethodChannel#flutter/platform( 9732):   at android.os.MessageQueue.nativePollOnce(Native Method)
2020-12-01T14:34:34.964571: stdout: [        ] E/MethodChannel#flutter/platform( 9732):   at android.os.MessageQueue.next(MessageQueue.java:323)
2020-12-01T14:34:34.964805: stdout: [        ] E/MethodChannel#flutter/platform( 9732):   at android.os.Looper.loop(Looper.java:136)
2020-12-01T14:34:34.965034: stdout: [        ] E/MethodChannel#flutter/platform( 9732):   at android.app.ActivityThread.main(ActivityThread.java:6123)
2020-12-01T14:34:34.965248: stdout: [        ] E/MethodChannel#flutter/platform( 9732):   at java.lang.reflect.Method.invoke(Native Method)
2020-12-01T14:34:34.965398: stdout: [        ] E/MethodChannel#flutter/platform( 9732):   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)
2020-12-01T14:34:34.965589: stdout: [        ] E/MethodChannel#flutter/platform( 9732):   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:757)
2020-12-01T14:34:34.966863: stderr: [   +1 ms] VMServiceFlutterDriver: <<< {isError: false, response: {}, type: _extensionType, method: ext.flutter.driver}
2020-12-01T14:34:34.967698: stderr: [        ] VMServiceFlutterDriver: >>> {command: get_text, finderType: ByValueKey, keyValueString: status, keyValueType: String}
2020-12-01T14:34:34.986399: stdout: [  +18 ms] E/flutter ( 9732): [ERROR:flutter/shell/common/shell.cc(209)] Dart Unhandled Exception: PlatformException(error, Attempt to invoke interface method 'boolean io.flutter.plugin.platform.PlatformPlugin$PlatformPluginDelegate.popSystemNavigator()' on a null object reference, null, java.lang.NullPointerException: Attempt to invoke interface method 'boolean io.flutter.plugin.platform.PlatformPlugin$PlatformPluginDelegate.popSystemNavigator()' on a null object reference
2020-12-01T14:34:34.986682: stdout: [        ] E/flutter ( 9732):   at io.flutter.plugin.platform.PlatformPlugin.popSystemNavigator(PlatformPlugin.java:313)
stdout: [        ] E/flutter ( 9732):   at io.flutter.plugin.platform.PlatformPlugin.access$600(PlatformPlugin.java:28)
2020-12-01T14:34:34.987013: stdout: [        ] E/flutter ( 9732):   at io.flutter.plugin.platform.PlatformPlugin$1.popSystemNavigator(PlatformPlugin.java:101)
2020-12-01T14:34:34.987328: stdout: [        ] E/flutter ( 9732):   at io.flutter.embedding.engine.systemchannels.PlatformChannel$1.onMethodCall(PlatformChannel.java:123)
2020-12-01T14:34:34.987547: stdout: [        ] E/flutter ( 9732):   at io.flutter.plugin.common.MethodChannel$IncomingMethodCallHandler.onMessage(MethodChannel.java:233)
2020-12-01T14:34:34.987815: stdout: [        ] E/flutter ( 9732):   at io.flutter.embedding.engine.dart.DartMessenger.handleMessageFromDart(DartMessenger.java:85)
2020-12-01T14:34:34.988037: stdout: [        ] E/flutter ( 9732):   at io.flutter.embedding.engine.FlutterJNI.handlePlatformMessage(FlutterJNI.java:738)
2020-12-01T14:34:34.988246: stdout: [        ] E/flutter ( 9732):   at android.os.MessageQueue.nativePollOnce(Native Method)
2020-12-01T14:34:34.988470: stdout: [        ] E/flutter ( 9732):   at android.os.MessageQueue.next(MessageQueue.java:323)
2020-12-01T14:34:34.988678: stdout: [        ] E/flutter ( 9732):   at android.os.Looper.loop(Looper.java:136)
2020-12-01T14:34:34.988934: stdout: [        ] E/flutter ( 9732):   at android.app.ActivityThread.main(ActivityThread.java:6123)
2020-12-01T14:34:34.989112: stdout: [        ] E/flutter ( 9732):   at java.lang.reflect.Method.invoke(Native Method)
2020-12-01T14:34:34.989348: stdout: [        ] E/flutter ( 9732):   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)
2020-12-01T14:34:34.990016: stdout: [        ] E/flutter ( 9732):   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:757)
stdout: [        ] E/flutter ( 9732): ), stack trace: #0      JSONMethodCodec.decodeEnvelope (package:flutter/src/services/message_codecs.dart:159:7)
stdout: [        ] E/flutter ( 9732): flutter#1      MethodChannel._invokeMethod (package:flutter/src/services/platform_channel.dart:159:18)
2020-12-01T14:34:34.990374: stdout: [        ] E/flutter ( 9732): <asynchronous suspension>
2020-12-01T14:34:34.990702: stdout: [        ] E/flutter ( 9732): flutter#2      SystemNavigator.pop (package:flutter/src/services/system_navigator.dart:34:5)
2020-12-01T14:34:34.990946: stdout: [        ] E/flutter ( 9732): <asynchronous suspension>
```

This reverts commit 6a84b02.

Author: cbracken

bin/internal/engine.version

@@ -1 +1 @@
-d2ad4419bb07ef4ee7bd401c1c3e0d6fd6187127
+20caf549695193a9920ca37752eca4f141bbef31

bin/internal/fuchsia-linux.version

@@ -1 +1 @@
-Bnaeivv073toX9buVcBemk-hh-JEa4SYA2t0zNHdZLkC
+gkfmiRsIlyNLnVg-SFHcHXZnn87Snu3dJKHupuuEK5AC

bin/internal/fuchsia-mac.version

@@ -1 +1 @@
-qpkZl0s5J-NwdmoguE8Zs6cNBNWCMLoBsAsR9-L60T8C
+36uDTGJQpFUlxJONCRuyF3fTEC68XzCi0vdeI-jlyEUC