[dart2js] Report an error if throw is used outside of catch expression without an argument.

lib/compiler/implementation/ssa/builder.dart

Index: lib/compiler/implementation/ssa/builder.dart
diff --git a/lib/compiler/implementation/ssa/builder.dart b/lib/compiler/implementation/ssa/builder.dart
index c3deabd100b724d52d9546d5d613a49114ed22b0..03109eaf8858d4b900a8216a9c460d719831d0de 100644
--- a/lib/compiler/implementation/ssa/builder.dart
+++ b/lib/compiler/implementation/ssa/builder.dart
@@ -3197,8 +3197,8 @@ class SsaBuilder extends ResolvedVisitor implements Visitor {
       HInstruction exception = rethrowableException;
       if (exception === null) {
         exception = graph.addConstantNull(constantSystem);
-        compiler.reportError(node,
-                             'throw without expression outside catch block');
+        compiler.internalError(
+            'rethrowableException should not be null', node: node);
       }
       close(new HThrow(exception, isRethrow: true));
     } else {