Update Process.exitCode setter to not use @OverRide (flutter#7434)

As of Dart SDK 1.22.0-dev.5.0, `Process.exitCode` is no longer
mutable (that SDK version picks up dart-lang/sdk@e5a16b1).
This change allows the tools code to pass analysis in sdk versions both
before and after that change, to allow for analysis against both the host and
target sdks.

packages/flutter_tools/lib/src/base/process_manager.dart

@@ -491,8 +491,10 @@ class _RecordingProcess implements Process {
   @override
   Future<int> get exitCode => delegate.exitCode;
 
-  @override
-  set exitCode(Future<int> exitCode) => delegate.exitCode = exitCode;
+  // TODO(tvolkert): Remove this once the dart sdk in both the target and
+  // the host have picked up dart-lang/sdk@e5a16b1
+  @override // ignore: OVERRIDE_ON_NON_OVERRIDING_SETTER
+  set exitCode(Future<int> exitCode) => throw new UnsupportedError('set exitCode');
 
   @override
   Stream<List<int>> get stdout {
@@ -827,7 +829,9 @@ class _ReplayProcess implements Process {
   @override
   Future<int> get exitCode => _exitCodeCompleter.future;
 
-  @override
+  // TODO(tvolkert): Remove this once the dart sdk in both the target and
+  // the host have picked up dart-lang/sdk@e5a16b1
+  @override // ignore: OVERRIDE_ON_NON_OVERRIDING_SETTER
   set exitCode(Future<int> exitCode) => throw new UnsupportedError('set exitCode');
 
   @override