Add a shrinking reduction for dead continuation parameters.

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

Index: pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
index 139b3a1a86abb53480ae6b0ce3a5f30d8a77f050..00e3252a9459f472ab3e1b3cd194ae86aab814af 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
@@ -630,6 +630,12 @@ class Parameter extends Primitive {
     super.hint = hint;
   }
 
+  // In addition to a parent pointer to the containing Continuation or
+  // FunctionDefinition, parameters have an index into the list of parameters
+  // bound by the parent.  This gives constant-time access to the continuation
+  // from the parent.
+  int parent_index;
+
   accept(Visitor visitor) => visitor.visitParameter(this);
 }
 
@@ -680,7 +686,7 @@ class FieldDefinition extends Node implements ExecutableDefinition {
 
   /// `true` if this field has no initializer.
   ///
-  /// If `true` [body] and [returnContinuation] are `null`.
+  /// If `true` [body] is `null`.
   ///
   /// This is different from a initializer that is `null`. Consider this class:
   ///
@@ -717,7 +723,7 @@ class RunnableBody extends InteriorNode {
 }
 
 /// A function definition, consisting of parameters and a body.  The parameters
-/// include a distinguished continuation parameter.
+/// include a distinguished continuation parameter (held by the body).
 class FunctionDefinition extends Node
     implements ExecutableDefinition {
   final FunctionElement element;
@@ -751,8 +757,7 @@ class FunctionDefinition extends Node
 
   /// Returns `true` if this function is abstract or external.
   ///
-  /// If `true`, [body] and [returnContinuation] are `null` and [localConstants]
-  /// is empty.
+  /// If `true`, [body] is `null` and [localConstants] is empty.
   bool get isAbstract => body == null;
 }
 
@@ -884,6 +889,7 @@ abstract class RecursiveVisitor extends Visitor {
   processRunnableBody(RunnableBody node) {}
   visitRunnableBody(RunnableBody node) {
     processRunnableBody(node);
+    visit(node.returnContinuation);
     visit(node.body);
   }
 
@@ -1063,7 +1069,7 @@ abstract class RecursiveVisitor extends Visitor {
   visitContinuation(Continuation node) {
     processContinuation(node);
     node.parameters.forEach(visitParameter);
-    visit(node.body);
+    if (node.body != null) visit(node.body);
   }
 
   // Conditions.