Take analysis options into account when computing the severity of a strong mode "error".

pkg/analyzer/lib/src/task/strong/checker.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.
 
 // TODO(jmesserly): this was ported from package:dev_compiler, and needs to be
 // refactored to fit into analyzer.
 library analyzer.src.task.strong.checker;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_resolution_map.dart';
 import 'package:analyzer/dart/ast/token.dart' show TokenType;
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
+import 'package:analyzer/source/error_processor.dart' show ErrorProcessor;
 import 'package:analyzer/src/dart/element/element.dart';
 import 'package:analyzer/src/dart/element/type.dart';
 import 'package:analyzer/src/error/codes.dart' show StrongModeCode;
 import 'package:analyzer/src/generated/engine.dart' show AnalysisOptionsImpl;
 import 'package:analyzer/src/generated/resolver.dart' show TypeProvider;
 import 'package:analyzer/src/generated/type_system.dart';
 import 'package:analyzer/src/summary/idl.dart';
 
 import 'ast_properties.dart';
 
@@ skipping 1004 matching lines @@
     _recordMessage(expr, errorCode, [from, to]);
     if (opAssign) {
       setImplicitAssignmentCast(expr, to);
     } else {
       setImplicitCast(expr, to);
     }
     _hasImplicitCasts = true;
   }
 
   void _recordMessage(AstNode node, ErrorCode errorCode, List arguments) {
-    var severity = errorCode.errorSeverity;
+    // Compute the right severity taking the analysis options into account.
+    // We construct a dummy error to make the common case where we end up
+    // ignoring the strong mode message cheaper.
+    var processor = ErrorProcessor.getProcessor(_options,
+        new AnalysisError.forValues(null, -1, 0, errorCode, null, null));
+    var severity =
+        (processor != null) ? processor.severity : errorCode.errorSeverity;

[Brian Wilkerson, 2017/03/29 13:44:22]

If this works, then I suppose it's fine, but I don't understand why this would
be necessary. The error processor runs after analysis has produced diagnostics
and before those diagnostics are made visible, so I would have expected this
work to have been done later.

Unless you know why the existing mechanism is not sufficient for this case,
please add a TODO comment to remind us to look into it later.

+
     if (severity == ErrorSeverity.ERROR) _failure = true;
     if (severity != ErrorSeverity.INFO || _options.strongModeHints) {
       int begin = node is AnnotatedNode
           ? node.firstTokenAfterCommentAndMetadata.offset
           : node.offset;
       int length = node.end - begin;
       var source = resolutionMap
           .elementDeclaredByCompilationUnit(node.root as CompilationUnit)
           .source;
       var error =
@@ skipping 497 matching lines @@
     var visited = new Set<InterfaceType>();
     do {
       visited.add(current);
       current.mixins.reversed.forEach(
           (m) => _checkIndividualOverridesFromClass(node, m, seen, true));
       _checkIndividualOverridesFromClass(node, current.superclass, seen, true);
       current = current.superclass;
     } while (!current.isObject && !visited.contains(current));
   }
 }