dart2js-kernel: Emit error trap for unimplemented features

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

Index: pkg/compiler/lib/src/ssa/builder_kernel.dart
diff --git a/pkg/compiler/lib/src/ssa/builder_kernel.dart b/pkg/compiler/lib/src/ssa/builder_kernel.dart
index 4c3e52956ef8d01515794c580b893b868a748c65..b84d1ad44002580e3afb5e98a38aa4a97f763d1c 100644
--- a/pkg/compiler/lib/src/ssa/builder_kernel.dart
+++ b/pkg/compiler/lib/src/ssa/builder_kernel.dart
@@ -380,8 +380,17 @@ class KernelSsaBuilder extends ir.Visitor with GraphBuilder {
 
   @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.