Fix bug in convertIntoBlockBody

I ran across this bug while testing to see what work was already done.
I didn't take the time to create an issue.

The bug is that an assist was being offered for `class C ^{}` to
convert the class body into a block. The result, for an empty block,
was just to add an extra space before the block body. For a non-empty
block, the result was to delete everything inside the block.

Change-Id: I0342b681ef67b5e659a00126a998d63d593223fd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/508702
Reviewed-by: Samuel Rawlins <srawlins@google.com>

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

@@ -82,10 +82,12 @@ class ConvertIntoBlockBody extends ResolvedCorrectionProducer {
     if (body == null) {
       return;
     }
-    var bodyRange = body.sourceRange;
-    await builder.addDartFileEdit(file, (builder) {
-      builder.addSimpleReplacement(bodyRange, ' {}');
-    });
+    if (body is EmptyClassBody || body is EmptyEnumBody) {
+      var bodyRange = body.sourceRange;
+      await builder.addDartFileEdit(file, (builder) {
+        builder.addSimpleReplacement(bodyRange, ' {}');
+      });
+    }
   }
 
   Future<void> _computeMissingFunctionBody(

pkg/analysis_server/test/src/services/correction/assist/convert_into_block_body_test.dart

@@ -80,6 +80,13 @@ class C {}
 ''');
   }
 
+  Future<void> test_container_class_block() async {
+    await resolveTestCode('''
+class C ^{}
+''');
+    await assertNoAssist();
+  }
+
   Future<void> test_container_enum() async {
     await resolveTestCode(
       '''
@@ -92,6 +99,16 @@ enum E {}
 ''');
   }
 
+  Future<void> test_container_enum_block() async {
+    await resolveTestCode(
+      '''
+enum E ^{}
+''',
+      ignore: [diag.enumWithoutConstants],
+    );
+    await assertNoAssist();
+  }
+
   Future<void> test_container_extension() async {
     await resolveTestCode('''
 extension E on int^;
@@ -101,6 +118,13 @@ extension E on int {}
 ''');
   }
 
+  Future<void> test_container_extension_block() async {
+    await resolveTestCode('''
+extension E on int ^{}
+''');
+    await assertNoAssist();
+  }
+
   Future<void> test_container_extensionType() async {
     await resolveTestCode('''
 extension type ^E(int i);
@@ -110,6 +134,13 @@ extension type E(int i) {}
 ''');
   }
 
+  Future<void> test_container_extensionType_block() async {
+    await resolveTestCode('''
+extension type E(int i) ^{}
+''');
+    await assertNoAssist();
+  }
+
   Future<void> test_container_mixin() async {
     await resolveTestCode('''
 mixin M^;
@@ -119,6 +150,13 @@ mixin M {}
 ''');
   }
 
+  Future<void> test_container_mixin_block() async {
+    await resolveTestCode('''
+mixin M ^{}
+''');
+    await assertNoAssist();
+  }
+
   Future<void> test_inExpression() async {
     await resolveTestCode('''
 void f() => ^123;