Add operators test to source_mapping_test.

pkg/compiler/lib/src/ssa/ssa_tracer.dart

Index: pkg/compiler/lib/src/ssa/ssa_tracer.dart
diff --git a/pkg/compiler/lib/src/ssa/ssa_tracer.dart b/pkg/compiler/lib/src/ssa/ssa_tracer.dart
index 0b50f8c242101723bc0709d7383e35eae6a6a5b0..830c6c772fb245459219accbee64831858e36d90 100644
--- a/pkg/compiler/lib/src/ssa/ssa_tracer.dart
+++ b/pkg/compiler/lib/src/ssa/ssa_tracer.dart
@@ -389,6 +389,10 @@ class HInstructionStringifier implements HVisitor<String> {
     return buffer.toString();
   }
 
+  String visitRef(HRef node) {
+    return 'Ref ${temporaryId(node.value)}';
+  }
+
   String visitReturn(HReturn node) => "Return ${temporaryId(node.inputs[0])}";
 
   String visitShiftLeft(HShiftLeft node) => handleInvokeBinary(node, '<<');