Revert "Remove an unused field from one of the CPS translation classes."

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

Index: pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
index 32c63d0317547fa3e36501dfe2ca67bbe3359011..2ebe34ba70d410db8862a642d21e0bc451535b9c 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
@@ -479,6 +479,8 @@ abstract class IrBuilder {
   // TODO(johnniwinther): Make these field final and remove the default values
   // when [IrBuilder] is a property of [IrBuilderVisitor] instead of a mixin.
 
+  final List<ir.Parameter> _parameters = <ir.Parameter>[];
+
   IrBuilderDelimitedState state;
 
   /// A map from variable indexes to their values.
@@ -560,18 +562,20 @@ abstract class IrBuilder {
     _enterScope(closureScope);
   }
 
-  void buildFunctionHeader(Iterable<Local> parameters,
-                           {ClosureScope closureScope,
-                            ClosureEnvironment env}) {
+  List<ir.Primitive> buildFunctionHeader(Iterable<Local> parameters,
+                                        {ClosureScope closureScope,
+                                         ClosureEnvironment env}) {
     _createThisParameter();
     _enterClosureEnvironment(env);
     _enterScope(closureScope);
     parameters.forEach(_createFunctionParameter);
+    return _parameters;
   }
 
   /// Creates a parameter for [local] and adds it to the current environment.
   ir.Parameter createLocalParameter(Local local) {
     ir.Parameter parameter = new ir.Parameter(local);
+    _parameters.add(parameter);
     environment.extend(local, parameter);
     return parameter;
   }
@@ -2186,6 +2190,7 @@ class DartIrBuilder extends IrBuilder {
 
   void _createFunctionParameter(Local parameterElement) {
     ir.Parameter parameter = new ir.Parameter(parameterElement);
+    _parameters.add(parameter);
     if (isInMutableVariable(parameterElement)) {
       state.functionParameters.add(getMutableVariable(parameterElement));
     } else {
@@ -2394,6 +2399,7 @@ class JsIrBuilder extends IrBuilder {
 
   void _createFunctionParameter(Local parameterElement) {
     ir.Parameter parameter = new ir.Parameter(parameterElement);
+    _parameters.add(parameter);
     state.functionParameters.add(parameter);
     ClosureLocation location = jsState.boxedVariables[parameterElement];
     if (location != null) {