Land 'Fix a crash related to obsolete factory syntax.'.

lib/compiler/implementation/tree/unparser.dart

Index: lib/compiler/implementation/tree/unparser.dart
diff --git a/lib/compiler/implementation/tree/unparser.dart b/lib/compiler/implementation/tree/unparser.dart
index 8eda6371f0e8af805c210d71917a21d30d29440d..a99854a42edaf8852d3c77674c8510e1b7aa1214 100644
--- a/lib/compiler/implementation/tree/unparser.dart
+++ b/lib/compiler/implementation/tree/unparser.dart
@@ -270,7 +270,8 @@ class Unparser implements Visitor {
     if (!node.isPrefix && !node.isIndex) visit(node.selector);
     if (spacesNeeded) sb.add(' ');
     // Also add a space for sequences like x + +1 and y - -y.
-    if (opString === '-' || opString === '+') {
+    // TODO(ahe): remove case for '+' when we drop the support for it.
+    if (node.argumentsNode != null && (opString === '-' || opString === '+')) {
       Token beginToken = node.argumentsNode.getBeginToken();
       if (beginToken !== null && beginToken.stringValue === opString) {
         sb.add(' ');