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

lib/compiler/implementation/ssa/builder.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.
 
 class Interceptors {
   Compiler compiler;
   Interceptors(Compiler this.compiler);
 
   SourceString mapOperatorToMethodName(Operator op) {
     String name = op.source.stringValue;
@@ skipping 3179 matching lines @@
     } else {
       close(attachPosition(new HReturn(value), node)).addSuccessor(graph.exit);
     }
   }
 
   visitThrow(Throw node) {
     if (node.expression === null) {
       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 {
       visit(node.expression);
       close(new HThrow(pop()));
     }
   }
 
   visitTypeAnnotation(TypeAnnotation node) {
     compiler.internalError('visiting type annotation in SSA builder',
@@ skipping 1177 matching lines @@
           new HSubGraphBlockInformation(elseBranch.graph));
 
     HBasicBlock conditionStartBlock = conditionBranch.block;
     conditionStartBlock.setBlockFlow(info, joinBlock);
     SubGraph conditionGraph = conditionBranch.graph;
     HIf branch = conditionGraph.end.last;
     assert(branch is HIf);
     branch.blockInformation = conditionStartBlock.blockFlow;
   }
 }