Throw an exception when invariant fails and report nice diagnostic.

dart/lib/compiler/implementation/ssa/codegen.dart

Index: dart/lib/compiler/implementation/ssa/codegen.dart
diff --git a/dart/lib/compiler/implementation/ssa/codegen.dart b/dart/lib/compiler/implementation/ssa/codegen.dart
index fdf877a754a08a2d1488902d9dd53081e471ac12..8e0a01cda98af0736a53a0e154cd8d20aba43fe7 100644
--- a/dart/lib/compiler/implementation/ssa/codegen.dart
+++ b/dart/lib/compiler/implementation/ssa/codegen.dart
@@ -2307,9 +2307,11 @@ abstract class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     world.registerIsCheck(type);
     Element element = type.element;
     if (identical(element.kind, ElementKind.TYPE_VARIABLE)) {
-      compiler.unimplemented("visitIs for type variables", instruction: node);
+      compiler.unimplemented("visitIs for type variables",
+                             instruction: node.expression);
     } else if (identical(element.kind, ElementKind.TYPEDEF)) {
-      compiler.unimplemented("visitIs for typedefs", instruction: node);
+      compiler.unimplemented("visitIs for typedefs",
+                             instruction: node.expression);
     }
     LibraryElement coreLibrary = compiler.coreLibrary;
     ClassElement objectClass = compiler.objectClass;