dart2js cps: Eliminate unary string concatenation.

pkg/compiler/lib/src/cps_ir/type_propagation.dart

Index: pkg/compiler/lib/src/cps_ir/type_propagation.dart
diff --git a/pkg/compiler/lib/src/cps_ir/type_propagation.dart b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
index d20178c440434d09042d737eed2dfef0b6920f1e..bf5658abbf73d3500dc7cb6a4151430945d45876 100644
--- a/pkg/compiler/lib/src/cps_ir/type_propagation.dart
+++ b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
@@ -2073,6 +2073,11 @@ class TransformingVisitor extends DeepRecursiveVisitor {
         if (argumentsWereRemoved) {
           node.arguments.removeWhere((ref) => ref == null);
         }
+        if (node.arguments.length == 1) {
+          Primitive input = node.arguments[0].definition;
+          node.replaceUsesWith(input);
+          input.useElementAsHint(node.hint);
+        }
         // TODO(asgerf): Rebalance nested StringConcats that arise from
         //               rewriting the + operator to StringConcat.
         break;