Support conditional expressions in analyzer2dart.

sdk/lib/_internal/compiler/implementation/cps_ir/cps_ir_builder_visitor.dart

Index: sdk/lib/_internal/compiler/implementation/cps_ir/cps_ir_builder_visitor.dart
diff --git a/sdk/lib/_internal/compiler/implementation/cps_ir/cps_ir_builder_visitor.dart b/sdk/lib/_internal/compiler/implementation/cps_ir/cps_ir_builder_visitor.dart
index 04413a8dbf52297cd9193e2c5d326574aed6e05a..294aa0804a3cf60fca9d7002d6c801c0cc030bdf 100644
--- a/sdk/lib/_internal/compiler/implementation/cps_ir/cps_ir_builder_visitor.dart
+++ b/sdk/lib/_internal/compiler/implementation/cps_ir/cps_ir_builder_visitor.dart
@@ -177,7 +177,13 @@ class IrBuilderVisitor extends ResolvedVisitor<ir.Primitive>
     });
   }
 
-  ir.Primitive visit(ast.Node node) => node.accept(this);
+  ir.Primitive visit(ast.Node node) => node != null ? node.accept(this) : null;
+
+  /// Returns a closure that takes an [IrBuilder] and visits [node] in its
+  /// context.
+  build(ast.Node node) {
+    return (IrBuilder builder) => withBuilder(builder, () => visit(node));
+  }
 
   // ==== Statements ====
   // Build(Block(stamements), C) = C'
@@ -376,19 +382,10 @@ class IrBuilderVisitor extends ResolvedVisitor<ir.Primitive>
   }
 
    visitIf(ast.If node) {
-    ir.Primitive condition = visit(node.condition);
-
-    void buildThenPart(IrBuilder thenBuilder) {
-      withBuilder(thenBuilder, () => visit(node.thenPart));
-    }
-
-    void buildElsePart(IrBuilder elseBuilder) {
-      if (node.hasElsePart) {
-        withBuilder(elseBuilder, () => visit(node.elsePart));
-      }
-    }
-
-    irBuilder.buildIf(condition, buildThenPart, buildElsePart);
+    irBuilder.buildIf(
+        visit(node.condition),
+        build(node.thenPart),
+        build(node.elsePart));
   }
 
   ir.Primitive visitLabeledStatement(ast.LabeledStatement node) {
@@ -646,48 +643,10 @@ class IrBuilderVisitor extends ResolvedVisitor<ir.Primitive>
 
   // ==== Expressions ====
   ir.Primitive visitConditional(ast.Conditional node) {
-    assert(irBuilder.isOpen);
-    ir.Primitive condition = visit(node.condition);
-
-    // The then and else expressions are delimited.
-    IrBuilder thenBuilder = new IrBuilder.delimited(irBuilder);
-    IrBuilder elseBuilder = new IrBuilder.delimited(irBuilder);
-    ir.Primitive thenValue =
-        withBuilder(thenBuilder, () => visit(node.thenExpression));
-    ir.Primitive elseValue =
-        withBuilder(elseBuilder, () => visit(node.elseExpression));
-
-    // Treat the values of the subexpressions as named values in the
-    // environment, so they will be treated as arguments to the join-point
-    // continuation.
-    assert(irBuilder.environment.length == thenBuilder.environment.length);
-    assert(irBuilder.environment.length == elseBuilder.environment.length);
-    thenBuilder.environment.extend(null, thenValue);
-    elseBuilder.environment.extend(null, elseValue);
-    JumpCollector jumps = new JumpCollector(null);
-    jumps.addJump(thenBuilder);
-    jumps.addJump(elseBuilder);
-    ir.Continuation joinContinuation =
-        irBuilder.createJoin(irBuilder.environment.length + 1, jumps);
-
-    // Build the term
-    //   let cont join(x, ..., result) = [] in
-    //   let cont then() = [[thenPart]]; join(v, ...) in
-    //   let cont else() = [[elsePart]]; join(v, ...) in
-    //     if condition (then, else)
-    ir.Continuation thenContinuation = new ir.Continuation([]);
-    ir.Continuation elseContinuation = new ir.Continuation([]);
-    thenContinuation.body = thenBuilder._root;
-    elseContinuation.body = elseBuilder._root;
-    irBuilder.add(new ir.LetCont(joinContinuation,
-            new ir.LetCont(thenContinuation,
-                new ir.LetCont(elseContinuation,
-                    new ir.Branch(new ir.IsTrue(condition),
-                                  thenContinuation,
-                                  elseContinuation)))));
-    return (thenValue == elseValue)
-        ? thenValue
-        : joinContinuation.parameters.last;
+    return irBuilder.buildConditional(
+        visit(node.condition),
+        build(node.thenExpression),
+        build(node.elseExpression));
   }
 
   // For all simple literals: