Make source information on conditions mandatory in CPS

pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart

Index: pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
diff --git a/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart b/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
index 368513776e3f8f60c17f7ab2debf03e2e6f3a201..799610165fe501c8dd958c97f834c5268be2691c 100644
--- a/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
+++ b/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
@@ -758,7 +758,10 @@ class StatementRewriter extends Transformer implements Pass {
     assert(isCompoundableBuiltin(e));
     if (e.arguments.length > 2) {
       assert(e.operator == BuiltinOperator.StringConcatenate);
-      return new ApplyBuiltinOperator(e.operator, e.arguments.skip(1).toList());
+      return new ApplyBuiltinOperator(
+          e.operator,
+          e.arguments.skip(1).toList(),
+          e.sourceInformation);
     } else {
       return e.arguments[1];
     }