Use UnaryOperator and BinaryOperator in ConstantSystem.

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 8c8668ef3e175d1c825919d372acb48eb18aa793..1d1a7f2eb8a25b4b8b6b6f0a08c01f810d4b1982 100644
--- a/pkg/compiler/lib/src/cps_ir/type_propagation.dart
+++ b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
@@ -522,7 +522,8 @@ class _TypePropagationVisitor<T> implements Visitor {
       if (opname == "unary-") {
         opname = "-";
       }
-      UnaryOperation operation = constantSystem.lookupUnary(opname);
+      UnaryOperation operation = constantSystem.lookupUnary(
+          UnaryOperator.parse(opname));
       if (operation != null) {
         result = operation.fold(lhs.constant);
       }
@@ -535,7 +536,8 @@ class _TypePropagationVisitor<T> implements Visitor {
         return;
       }
 
-      BinaryOperation operation = constantSystem.lookupBinary(opname);
+      BinaryOperation operation = constantSystem.lookupBinary(
+          BinaryOperator.parse(opname));
       if (operation != null) {
         result = operation.fold(lhs.constant, rhs.constant);
       }