dart2js-kernel: Emit error trap for unimplemented features

pkg/compiler/lib/src/ssa/builder_kernel.dart

 // Copyright (c) 2016, 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.
 
 import 'package:kernel/ast.dart' as ir;
 
 import '../common.dart';
 import '../common/codegen.dart' show CodegenRegistry, CodegenWorkItem;
 import '../common/names.dart';
 import '../common/tasks.dart' show CompilerTask;
@@ skipping 362 matching lines @@
   }
 
   /// Pushes a boolean checking [expression] against null.
   pushCheckNull(HInstruction expression) {
     push(new HIdentity(
         expression, graph.addConstantNull(compiler), null, backend.boolType));
   }
 
   @override
   void defaultExpression(ir.Expression expression) {
-    // TODO(het): This is only to get tests working
-    stack.add(graph.addConstantNull(compiler));
+    // TODO(het): This is only to get tests working.
+    String message = 'Unhandled ir.${expression.runtimeType}  $expression';
+    HInstruction nullValue = graph.addConstantNull(compiler);
+    HInstruction errorMessage =
+        graph.addConstantString(new DartString.literal(message), compiler);
+    HInstruction trap = new HForeignCode(
+        js.js.parseForeignJS("#.#"),
+        backend.dynamicType,
+        <HInstruction>[nullValue, errorMessage]);
+    trap.sideEffects..setAllSideEffects()..setDependsOnSomething();
+    push(trap);
   }
 
   /// Returns the current source element.
   ///
   /// The returned element is a declaration element.
   // TODO(efortuna): Update this when we implement inlining.
   @override
   Element get sourceElement => astAdapter.getElement(target);
 
   @override
@@ skipping 1271 matching lines @@
     push(new HNot(popBoolified(), backend.boolType));
   }
 
   @override
   void visitStringConcatenation(ir.StringConcatenation stringConcat) {
     KernelStringBuilder stringBuilder = new KernelStringBuilder(this);
     stringConcat.accept(stringBuilder);
     stack.add(stringBuilder.result);
   }
 }