Add support for assert statements with messages to the analyzer.

pkg/analyzer/lib/src/generated/testing/ast_factory.dart

Index: pkg/analyzer/lib/src/generated/testing/ast_factory.dart
diff --git a/pkg/analyzer/lib/src/generated/testing/ast_factory.dart b/pkg/analyzer/lib/src/generated/testing/ast_factory.dart
index 86f1e049ba364d67706b6b12b65861cc92a4a311..f85d37a27aff03adc63cd04e6cb6f61dbe0f9cfe 100644
--- a/pkg/analyzer/lib/src/generated/testing/ast_factory.dart
+++ b/pkg/analyzer/lib/src/generated/testing/ast_factory.dart
@@ -46,11 +46,14 @@ class AstFactory {
       new AsExpression(
           expression, TokenFactory.tokenFromKeyword(Keyword.AS), type);
 
-  static AssertStatement assertStatement(Expression condition) =>
-      new AssertStatement(
+  static AssertStatement assertStatement(Expression condition,
+          [Expression message]) =>
+      new AssertStatement.withOptionalMessage(
           TokenFactory.tokenFromKeyword(Keyword.ASSERT),
           TokenFactory.tokenFromType(TokenType.OPEN_PAREN),
           condition,
+          message == null ? null : TokenFactory.tokenFromType(TokenType.COMMA),
+          message,
           TokenFactory.tokenFromType(TokenType.CLOSE_PAREN),
           TokenFactory.tokenFromType(TokenType.SEMICOLON));