Correctly indent the RHS of 'is' and 'as' expressions.

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 9c4cd2c6d3308413fbbbc772cf8fe552d46966ff..f10cdd07544b298eba08b56a86ffa36afc76006d 100644
--- a/lib/src/source_visitor.dart
+++ b/lib/src/source_visitor.dart
@@ -159,11 +159,13 @@ class SourceVisitor implements AstVisitor {
 
   visitAsExpression(AsExpression node) {
     builder.startSpan();
+    builder.nestExpression();
     visit(node.expression);
     soloSplit();
     token(node.asOperator);
     space();
     visit(node.type);
+    builder.unnest();
     builder.endSpan();
   }
 
@@ -1240,12 +1242,14 @@ class SourceVisitor implements AstVisitor {
 
   visitIsExpression(IsExpression node) {
     builder.startSpan();
+    builder.nestExpression();
     visit(node.expression);
     soloSplit();
     token(node.isOperator);
     token(node.notOperator);
     space();
     visit(node.type);
+    builder.unnest();
     builder.endSpan();
   }