Fix interpolation unparse.

lib/compiler/implementation/scanner/listener.dart

Index: lib/compiler/implementation/scanner/listener.dart
diff --git a/lib/compiler/implementation/scanner/listener.dart b/lib/compiler/implementation/scanner/listener.dart
index 79f6cd595a24d552932272a359bbeeedeedc302f..2c6bd2377d52c277fe55a599310486bb91568a22 100644
--- a/lib/compiler/implementation/scanner/listener.dart
+++ b/lib/compiler/implementation/scanner/listener.dart
@@ -871,7 +871,8 @@ class ElementListener extends Listener {
     if (isLast) {
       pushNode(string);
     } else {
-      NodeList partNodes = new NodeList(null, parts, null, null);
+      NodeList partNodes =
+          new NodeList(null, parts, null, const SourceString(""));
       pushNode(new StringInterpolation(string, partNodes));
     }
   }