* DropdownButton should display the value over hint and disabledHint if a DropdownButton.value is defined

In principle this is backwards compatible; I just merged all the classes into one and deprecated everything that became redundant as a result.

Create symlinks for local engine Maven metadata files required by the Android Gradle plugin (#71600)

This reverts commit d047d108.

This reverts commit 16dce765.

This reverts commit 59cf0d22.

This reverts commit d688b52a.

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): #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): #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 6a84b02f.

* d6beaed56 Roll Fuchsia Linux SDK from gkfmiRsIl... to un3JixwuO... (flutter/engine#22744)

* 8832b480e Roll Skia from 888c5d3e57eb to 51b74afb84d4 (12 revisions) (flutter/engine#22746)

* e89090181 Don't register CanvasKit with `define` (flutter/engine#22745)

* 3c5167923 Roll Skia from 51b74afb84d4 to 452369182f6e (1 revision) (flutter/engine#22749)

* 5bf653353 Introduce a delegate class for gpu metal rendering (flutter/engine#22611)

* 5131aa40f Roll Skia from 452369182f6e to f2efb80bc316 (4 revisions) (flutter/engine#22750)

* 7b5f79f1a fuchsia: Ensure full-screen input interceptor (flutter/engine#22687)

* cec8a6e26 Manual roll of Dart SDK from ce76503f5b46 to dcd5a8f005a (flutter/engine#22766)

* 001a51193 Roll Fuchsia Linux SDK from un3JixwuO... to Bnaeivv07... (flutter/engine#22757)

* b9615b1d3 Roll Fuchsia Mac SDK from 36uDTGJQp... to qpkZl0s5J... (flutter/engine#22753)

* c4c476360 Roll Skia from f2efb80bc316 to 8d78da910e45 (5 revisions) (flutter/engine#22754)

* dbd1abec1 Roll Dart SDK from dcd5a8f005a2 to 960620d2e811 (794 revisions) (flutter/engine#22768)

* 1c2a6bd0a Fix the unchecked conversion warning for searchPaths in PlayStoreDynamicFeatureManager (flutter/engine#22654)

* 81af789ed add file package to deps in prep for glob update (flutter/engine#22770)

* a35e3fe4a Let FlutterFragment not pop the whole activity by default when more fragments are in the activity (flutter/engine#22692)

* adb3312f0 Revert "Introduce a delegate class for gpu metal rendering (#22611)" (flutter/engine#22775)

* bcc8832c1 Cleanup dart_runner examples & tests. (flutter/engine#22769)

* 609307de1 Roll Skia from 8d78da910e45 to fd41d878b13d (20 revisions) (flutter/engine#22772)

* 587c023ab [web] Add new line break type (prohibited) (flutter/engine#22771)

* 6b2ed2b5e Roll Skia from fd41d878b13d to 70fe17e12f38 (6 revisions) (flutter/engine#22776)

* 7910a1732 Roll Dart SDK from 960620d2e811 to 7a2a3968ef53 (12 revisions) (flutter/engine#22778)

* f4ada800d Roll Skia from 70fe17e12f38 to 4c6f57a23e63 (1 revision) (flutter/engine#22781)

* 3101dff34 [web] Optimize Matrix4.identity (flutter/engine#22622)

* a4ce848df Add FlutterPlayStoreSplitApplication for simpler opt in to Split AOT (flutter/engine#22752)

* 747b7912c Add file.dart to DEPS (flutter/engine#22794)

* 40fa345c0 Fix race condition in key event handling on Android (flutter/engine#22658)

* d2ad4419b Fix PlatformDispatcher.locale to return something meaningful when there are no locales. (flutter/engine#22608)