Introduce benign errors.

Closes #25204

R=sigmund@google.com

Review URL: https://codereview.chromium.org/1525593002.

pkg/compiler/lib/src/compiler.dart

@@ -1250,6 +1250,32 @@ abstract class Compiler {
     _reporter.onCrashInUserCode(message, exception, stackTrace);
   }
 
+  /// Messages for which compile-time errors are reported but compilation
+  /// continues regardless.
+  static const List<MessageKind> BENIGN_ERRORS = const <MessageKind>[
+      MessageKind.INVALID_METADATA,
+      MessageKind.INVALID_METADATA_GENERIC,
+  ];
+
+  bool markCompilationAsFailed(DiagnosticMessage message, api.Diagnostic kind) {
+    if (testMode) {
+      // When in test mode, i.e. on the build-bot, we always stop compilation.
+      return true;
+    }
+    if (reporter.options.fatalWarnings) {
+      return true;
+    }
+    return !BENIGN_ERRORS.contains(message.message.kind);
+  }
+
+  void fatalDiagnosticReported(DiagnosticMessage message,
+                               List<DiagnosticMessage> infos,
+                               api.Diagnostic kind) {
+    if (markCompilationAsFailed(message, kind)) {
+      compilationFailed = true;
+    }
+  }
+
   /**
    * Translates the [resolvedUri] into a readable URI.
    *
@@ -1701,13 +1727,13 @@ class _CompilerDiagnosticReporter extends DiagnosticReporter {
   void reportDiagnostic(DiagnosticMessage message,
                         List<DiagnosticMessage> infos,
                         api.Diagnostic kind) {
+    compiler.reportDiagnostic(message, infos, kind);
     if (kind == api.Diagnostic.ERROR ||
         kind == api.Diagnostic.CRASH ||
         (options.fatalWarnings &&
          kind == api.Diagnostic.WARNING)) {
-      compiler.compilationFailed = true;
+      compiler.fatalDiagnosticReported(message, infos, kind);
     }
-    compiler.reportDiagnostic(message, infos, kind);
   }
 
   /**

tests/compiler/dart2js/benign_error_test.dart

@@ -0,0 +1,40 @@
+// 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.
+
+// Test that benign error do not prevent compilation.
+
+import 'memory_compiler.dart';
+
+import 'package:async_helper/async_helper.dart';
+import 'package:compiler/src/compiler.dart';
+import 'package:compiler/src/diagnostics/messages.dart';
+import 'package:compiler/src/js_backend/js_backend.dart';
+import 'package:expect/expect.dart';
+
+main() {
+  asyncTest(() async {
+    for (MessageKind kind in Compiler.BENIGN_ERRORS) {
+      await testExamples(kind);
+    }
+  });
+}
+
+testExamples(MessageKind kind) async {
+  MessageTemplate template = MessageTemplate.TEMPLATES[kind];
+  for (var example in template.examples) {
+    if (example is! Map) {
+      example = {'main.dart': example};
+    }
+    DiagnosticCollector collector = new DiagnosticCollector();
+    CompilationResult result = await runCompiler(
+        memorySourceFiles: example,
+        diagnosticHandler: collector);
+    Expect.isTrue(result.isSuccess);
+    Expect.isTrue(
+        collector.errors.any((message) => message.messageKind == kind));
+    Compiler compiler = result.compiler;
+    JavaScriptBackend backend = compiler.backend;
+    Expect.isNotNull(backend.generatedCode[compiler.mainFunction]);
+  }
+}