Don't specify 'void' return type for setters in 'Encapsulate Field'.

R=brianwilkerson@google.com

Bug: https://github.com/flutter/flutter-intellij/issues/2104
Change-Id: Ib7fe60e27f37726be5131c8a2e915b7911773855
Reviewed-on: https://dart-review.googlesource.com/52180
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analysis_server/lib/src/services/correction/assist_internal.dart

@@ -1240,7 +1240,7 @@ class AssistProcessor {
           : '';
       String getterCode = '$eol2  ${typeNameCode}get $name => _$name;';
       String setterCode = '$eol2'
-          '  void set $name($typeNameCode$name) {$eol'
+          '  set $name($typeNameCode$name) {$eol'
           '    _$name = $name;$eol'
           '  }';
       builder.addSimpleInsertion(fieldDeclaration.end, getterCode + setterCode);

pkg/analysis_server/test/services/correction/assist_test.dart

@@ -2578,7 +2578,7 @@ class A {
 
   int get test => _test;
 
-  void set test(int test) {
+  set test(int test) {
     _test = test;
   }
   A(this._test);
@@ -2604,7 +2604,7 @@ class A {
 
   get test => _test;
 
-  void set test(test) {
+  set test(test) {
     _test = test;
   }
 }