Handle metadata annotations in parameter lists with trailing commas.

lib/src/source_visitor.dart

Index: lib/src/source_visitor.dart
diff --git a/lib/src/source_visitor.dart b/lib/src/source_visitor.dart
index cb19267087c469c76bb073ed291b51313813dee4..a8e782bc9f0a29d7eeac334636c6ad3c90ae2273 100644
--- a/lib/src/source_visitor.dart
+++ b/lib/src/source_visitor.dart
@@ -2037,6 +2037,8 @@ class SourceVisitor implements AstVisitor {
     // Can't have a trailing comma if there are no parameters.
     assert(parameters.parameters.isNotEmpty);
 
+    _metadataRules.add(new MetadataRule());
+
     // Always split the parameters.
     builder.startRule(new Rule.hard());
 
@@ -2061,7 +2063,6 @@ class SourceVisitor implements AstVisitor {
     builder = builder.startBlock(null);
 
     for (var parameter in parameters.parameters) {
-      builder.nestExpression();
       visit(parameter);
 
       // The comma after the parameter.
@@ -2077,7 +2078,6 @@ class SourceVisitor implements AstVisitor {
         lastRequired = null;
       }
 
-      builder.unnest();
       newline();
     }
 
@@ -2088,6 +2088,8 @@ class SourceVisitor implements AstVisitor {
     builder = builder.endBlock(null, forceSplit: true);
     builder.endRule();
 
+    _metadataRules.removeLast();
+
     // Now write the delimiter itself.
     _writeText(firstDelimiter.lexeme, firstDelimiter.offset);
     if (firstDelimiter != parameters.rightParenthesis) {