cps-ir: Implement boolean conversion in and use it to implement '&&', '||', and '?:'.

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 9ad19f0ca69c684832b87e506b750b77f62b5028..64e6571eeb31d251aff9bd33743ff5c947290027 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
@@ -580,6 +580,8 @@ List<Reference<Primitive>> _referenceList(Iterable<Primitive> definitions) {
 }
 
 abstract class Visitor<T> {
+  const Visitor();
+
   T visit(Node node) => node.accept(this);
   // Abstract classes.
   T visitNode(Node node) => null;
@@ -623,6 +625,8 @@ abstract class Visitor<T> {
 /// Recursively visits the entire CPS term, and calls abstract `process*`
 /// (i.e. `processLetPrim`) functions in pre-order.
 abstract class RecursiveVisitor extends Visitor {
+  const RecursiveVisitor();
+
   // Ensures that RecursiveVisitor contains overrides for all relevant nodes.
   // As a rule of thumb, nodes with structure to traverse should be overridden
   // with the appropriate visits in this class (for example, visitLetCont),