Support unreachable code in blocks 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 9b20c5e6f4031be1c5404f937df7afe2ca576f29..1489aa3ccf801822cf82d4612c84d7a392d1322e 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
@@ -180,15 +180,8 @@ class IrBuilderVisitor extends ResolvedVisitor<ir.Primitive>
   ir.Primitive visit(ast.Node node) => node.accept(this);
 
   // ==== Statements ====
-  // Build(Block(stamements), C) = C'
-  //   where C' = statements.fold(Build, C)
-  ir.Primitive visitBlock(ast.Block node) {
-    assert(irBuilder.isOpen);
-    for (ast.Node n in node.statements.nodes) {
-      visit(n);
-      if (!irBuilder.isOpen) return null;
-    }
-    return null;
+  visitBlock(ast.Block node) {
+    irBuilder.buildBlock(node.statements.nodes, build);
   }
 
   ir.Primitive visitBreakStatement(ast.BreakStatement node) {