Augment. Remove a few 'abstract' modifiers from classes.

These were previously added because the parser did not support
abstract fields without diagnostics, IIRC.

Change-Id: Iba654bbfba3828e9823a65cc2b28f11a9ae0144a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/508441
Reviewed-by: Johnni Winther <johnniwinther@google.com>

pkg/analyzer/test/src/diagnostics/augmentation_without_declaration_test.dart

@@ -111,13 +111,13 @@ augment class A {
 
   test_class_instanceField_augments_instanceField_final() async {
     await resolveTestCodeWithDiagnostics(r'''
-abstract class A {
+class A {
   final int foo = 0;
 //          ^^^
 // [context 1] The corresponding getter is induced by this declaration.
 }
 
-augment abstract class A {
+augment class A {
   augment abstract int foo;
 //                     ^^^
 // [diag.augmentationWithoutSetterDeclaration][context 1] This augmentation induces a setter, but no setter declaration named 'foo' exists to augment.
@@ -127,13 +127,13 @@ augment abstract class A {
 
   test_class_instanceField_augments_instanceGetter() async {
     await resolveTestCodeWithDiagnostics(r'''
-abstract class A {
+class A {
   int get foo => 0;
 //        ^^^
 // [context 1] The corresponding getter is declared here.
 }
 
-augment abstract class A {
+augment class A {
   augment abstract int foo;
 //                     ^^^
 // [diag.augmentationWithoutSetterDeclaration][context 1] This augmentation induces a setter, but no setter declaration named 'foo' exists to augment.
@@ -250,13 +250,13 @@ augment class A {
 
   test_class_instanceField_final_augments_instanceSetter() async {
     await resolveTestCodeWithDiagnostics(r'''
-abstract class A {
+class A {
   set foo(int _) {}
 //    ^^^
 // [context 1] The corresponding setter is declared here.
 }
 
-augment abstract class A {
+augment class A {
   augment abstract final int foo;
 //                           ^^^
 // [diag.augmentationWithoutGetterDeclaration][context 1] This augmentation induces a getter, but no getter declaration named 'foo' exists to augment.

pkg/analyzer/test/src/diagnostics/duplicate_definition_test.dart

@@ -154,13 +154,13 @@ class C {
 
   test_instance_getter_field_augment() async {
     await resolveTestCodeWithDiagnostics(r'''
-abstract class C {
+class C {
   int get foo;
 //        ^^^
 // [context 1] The corresponding getter is declared here.
 }
 
-augment abstract class C {
+augment class C {
   augment int foo = 0;
 //            ^^^
 // [diag.augmentationWithoutSetterDeclaration][context 1] This augmentation induces a setter, but no setter declaration named 'foo' exists to augment.
@@ -459,13 +459,13 @@ class A(var int _, var int _);
 
   test_instance_setter_field_augment() async {
     await resolveTestCodeWithDiagnostics(r'''
-abstract class C {
+class C {
   void set foo(int _);
 //         ^^^
 // [context 1] The corresponding setter is declared here.
 }
 
-augment abstract class C {
+augment class C {
   augment int foo = 0;
 //            ^^^
 // [diag.augmentationWithoutGetterDeclaration][context 1] This augmentation induces a getter, but no getter declaration named 'foo' exists to augment.