Prepare for change in handling of NoSuchMethod.

In a future release of Dart, if a class containing private members is
*implemented* (rather than *extendeed*), any attempt to invoke one of
the private members will result in a runtime error (see
dart-lang/sdk#49687).  This is necessary in
order to soundly support field promotion
(dart-lang/language#2020).

The runtime error will most likely be a new exception type, not
`NoSuchMethodError`.  So, to avoid breaking the
`SharedPreferencesStorePlatform.instance` setter, we need to
generalize it to consider any exception thrown by
`_verifyProvidesDefaultImplementations` as an indication that the
`SharedPreferencesStorePlatform` has been illegally implemented.

In the non-error case, `_verifyProvidesDefaultImplementations` does
nothing, so this generalization should be safe.

Author: stereotype441

packages/shared_preferences/shared_preferences_platform_interface/lib/shared_preferences_platform_interface.dart

@@ -27,7 +27,7 @@ abstract class SharedPreferencesStorePlatform {
     if (!value.isMock) {
       try {
         value._verifyProvidesDefaultImplementations();
-      } on NoSuchMethodError catch (_) {
+      } catch (_) {
         throw AssertionError(
             'Platform interfaces must not be implemented with `implements`');
       }

packages/shared_preferences/shared_preferences_platform_interface/test/shared_preferences_platform_interface_test.dart

@@ -17,6 +17,15 @@ void main() {
         throwsAssertionError,
       );
     });
+
+    test('disallows implementing interface, unusual exception', () {
+      expect(
+        () {
+          SharedPreferencesStorePlatform.instance = IllegalImplementationWithUnusualException();
+        },
+        throwsAssertionError,
+      );
+    });
   });
 }
 
@@ -46,3 +55,12 @@ class IllegalImplementation implements SharedPreferencesStorePlatform {
     throw UnimplementedError();
   }
 }
+
+class IllegalImplementationWithUnusualException extends IllegalImplementation {
+  @override
+  dynamic noSuchMethod(Invocation invocation) {
+    throw _UnusualException();
+  }
+}
+
+class _UnusualException {}