dart2js: Remove rethrow from the CPS and Tree IRs.

pkg/compiler/lib/src/js_backend/codegen/codegen.dart

Index: pkg/compiler/lib/src/js_backend/codegen/codegen.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
index ab412b2a604540d4e1f803e4c02afe88d5e42533..bc8ed93511e60d6b00bfbe1c2f38bc95a481ac47 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
@@ -789,11 +789,6 @@ class CodeGenerator extends tree_ir.StatementVisitor
   }
 
   @override
-  void visitRethrow(tree_ir.Rethrow node) {
-    glue.reportInternalError('rethrow seen in JavaScript output');
-  }
-
-  @override
   void visitUnreachable(tree_ir.Unreachable node) {
     if (emitUnreachableAsReturn.last) {
       // Emit a return to assist local analysis in the VM.