Don't upgrade the severity of *everything* to an error in strong mode.

pkg/analyzer/lib/source/error_processor.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.source.error_processor;
 
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/error.dart';
 import 'package:analyzer/src/generated/utilities_general.dart';
 import 'package:analyzer/src/task/options.dart';
@@ skipping 76 matching lines @@
   bool appliesTo(AnalysisError error) => code == error.errorCode.name;
 
   /// Return an error processor associated with this [context] for the given
   /// [error], or `null` if none is found.
   static ErrorProcessor getProcessor(
       AnalysisContext context, AnalysisError error) {
     if (context == null) {
       return null;
     }
 
-    // By default, the error is not processed.
-    ErrorProcessor processor;
-
-    // Give strong mode a chance to upgrade it.
-    if (context.analysisOptions.strongMode) {
-      processor = _StrongModeTypeErrorProcessor.instance;
-    }
-
     // Let the user configure how specific errors are processed.
     List<ErrorProcessor> processors =
         context.getConfigurationData(CONFIGURED_ERROR_PROCESSORS);
 
+    // Give strong mode a chance to upgrade it.
+    if (context.analysisOptions.strongMode) {
+      processors = processors.toList();

[Brian Wilkerson, 2016/03/17 19:54:14]

It looks like it should already be a list. Is 'getConfigurationData' returning
something other than a list (in which case the 'toList' might want to be moved
up a couple of lines)?

[Bob Nystrom, 2016/03/17 19:58:25]

It is a list, but I want to copy it so I don't mutate the original one with the
.add() call.

+      processors.add(_StrongModeTypeErrorProcessor.instance);
+    }
+
     return processors.firstWhere((ErrorProcessor p) => p.appliesTo(error),
-        orElse: () => processor);
+        orElse: () => null);
   }
 }
 
 /// In strong mode, this upgrades static type warnings to errors.
 class _StrongModeTypeErrorProcessor implements ErrorProcessor {
   static final instance = new _StrongModeTypeErrorProcessor();
 
   // TODO(rnystrom): As far as I know, this is only used to implement
   // appliesTo(). Consider making it private in ErrorProcessor if possible.
   String get code => throw new UnsupportedError(
       "_StrongModeTypeErrorProcessor is not specific to an error code.");
 
   /// In strong mode, type warnings are upgraded to errors.
   ErrorSeverity get severity => ErrorSeverity.ERROR;
 
   /// Check if this processor applies to the given [error].
   bool appliesTo(AnalysisError error) =>
       error.errorCode.type == ErrorType.STATIC_TYPE_WARNING;
 }