Add CPS IR transformation to make the JavaScript backend specific semantics explicit in the tree.

pkg/compiler/lib/src/js_backend/codegen/unsugar.dart

+library dart2js.unsugar_cps;
+
+import '../../cps_ir/cps_ir_nodes.dart';
+
+// TODO(karlklose): share the [ParentVisitor].
+import '../../cps_ir/optimizers.dart';
+import '../../constants/expressions.dart';
+import '../../constants/values.dart';
+
+/// Rewrites the initial CPS IR to make Dart semantics explicit and inserts
+/// special nodes that respect JavaScript behavior.
+///
+/// Performs the following rewrites:
+///  - rewrite [IsTrue] in a [Branch] to do boolean conversion.
+class UnsugarVisitor extends RecursiveVisitor {
+  const UnsugarVisitor();
+
+  void rewrite(FunctionDefinition function) {
+    // Set all parent pointers.
+    new ParentVisitor().visit(function);
+    visit(function);
+  }
+
+  @override
+  visit(Node node) {
+    Node result = node.accept(this);
+    return result != null ? result : node;
+  }
+
+  Constant get trueConstant {
+    return new Constant(
+        new PrimitiveConstantExpression(
+            new TrueConstantValue()));
+  }
+
+  processBranch(Branch node) {
+    // TODO(karlklose): implement the checked mode part of boolean conversion.
+    InteriorNode parent = node.parent;
+    IsTrue condition = node.condition;
+    Primitive t = trueConstant;
+    Primitive i = new Identical(condition.value.definition, t);
+    LetPrim newNode = new LetPrim(t,
+        new LetPrim(i,
+            new Branch(new IsTrue(i),
+                node.trueContinuation.definition,
+                node.falseContinuation.definition)));
+    condition.value.unlink();
+    node.trueContinuation.unlink();
+    node.falseContinuation.unlink();
+    parent.body = newNode;
+  }
+}