Throw an exception when invariant fails and report nice diagnostic.

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of ssa;
 
 class SsaCodeGeneratorTask extends CompilerTask {
 
   final JavaScriptBackend backend;
 
@@ skipping 2289 matching lines @@
     push(new js.Binary('&&',
                        objectTest,
                        new js.Binary('||', arrayTest, pop())));
   }
 
   void visitIs(HIs node) {
     DartType type = node.typeExpression;
     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;
     HInstruction input = node.expression;
 
     if (identical(element, objectClass) || identical(element, compiler.dynamicClass)) {
       // The constant folder also does this optimization, but we make
       // it safe by assuming it may have not run.
       push(new js.LiteralBool(true), node);
     } else if (element == compiler.stringClass) {
@@ skipping 669 matching lines @@
   if (leftType.canBeNull() && rightType.canBeNull()) {
     if (left.isConstantNull() || right.isConstantNull() ||
         (leftType.isPrimitive() && leftType == rightType)) {
       return '==';
     }
     return null;
   } else {
     return '===';
   }
 }