Revert "dart2js: Constant fold num.round()"

pkg/compiler/lib/src/constant_system_dart.dart

Index: pkg/compiler/lib/src/constant_system_dart.dart
diff --git a/pkg/compiler/lib/src/constant_system_dart.dart b/pkg/compiler/lib/src/constant_system_dart.dart
index e1d468df5e05de6363e5362ab8b0d18fbb56f547..0f9a0d6225185ee4b6755eb6473fe564bc29324c 100644
--- a/pkg/compiler/lib/src/constant_system_dart.dart
+++ b/pkg/compiler/lib/src/constant_system_dart.dart
@@ -348,13 +348,20 @@ class IfNullOperation implements BinaryOperation {
   apply(left, right) => left ?? right;
 }
 
-class CodeUnitAtOperation implements BinaryOperation {
-  String get name => 'charCodeAt';
+abstract class CodeUnitAtOperation implements BinaryOperation {
+  final String name = 'charCodeAt';
   const CodeUnitAtOperation();
-  ConstantValue fold(ConstantValue left, ConstantValue right) => null;
   apply(left, right) => left.codeUnitAt(right);
 }
 
+class CodeUnitAtConstantOperation extends CodeUnitAtOperation {
+  const CodeUnitAtConstantOperation();
+  ConstantValue fold(ConstantValue left, ConstantValue right) {
+    // 'a'.codeUnitAt(0) is not a constant expression.
+    return null;
+  }
+}
+
 class CodeUnitAtRuntimeOperation extends CodeUnitAtOperation {
   const CodeUnitAtRuntimeOperation();
   IntConstantValue fold(ConstantValue left, ConstantValue right) {
@@ -372,14 +379,6 @@ class CodeUnitAtRuntimeOperation extends CodeUnitAtOperation {
   }
 }
 
-class UnfoldedUnaryOperation implements UnaryOperation {
-  final String name;
-  const UnfoldedUnaryOperation(this.name);
-  ConstantValue fold(ConstantValue constant) {
-    return null;
-  }
-}
-
 /**
  * A constant system implementing the Dart semantics. This system relies on
  * the underlying runtime-system. That is, if dart2js is run in an environment
@@ -410,8 +409,7 @@ class DartConstantSystem extends ConstantSystem {
   final shiftRight = const ShiftRightOperation();
   final subtract = const SubtractOperation();
   final truncatingDivide = const TruncatingDivideOperation();
-  final codeUnitAt = const CodeUnitAtOperation();
-  final round = const UnfoldedUnaryOperation('round');
+  final codeUnitAt = const CodeUnitAtConstantOperation();
 
   const DartConstantSystem();