Nicer error for JS() form with mismatched arguments.

pkg/compiler/lib/src/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.
 
 part of ssa;
 
 class SsaFunctionCompiler implements FunctionCompiler {
   final SsaCodeGeneratorTask generator;
   final SsaBuilderTask builder;
   final SsaOptimizerTask optimizer;
@@ skipping 4056 matching lines @@
       _) {
     generateCallInvoke(node, localsHandler.readLocal(function),
         sourceInformationBuilder.buildCall(node, node.argumentsNode));
   }
 
   void handleForeignJs(ast.Send node) {
     Link<ast.Node> link = node.arguments;
     // Don't visit the first argument, which is the type, and the second
     // argument, which is the foreign code.
     if (link.isEmpty || link.tail.isEmpty) {
+      // We should not get here because the call should be compiled to NSM.
       compiler.internalError(node.argumentsNode,
           'At least two arguments expected.');
     }
     native.NativeBehavior nativeBehavior =
         compiler.enqueuer.resolution.nativeEnqueuer.getNativeBehaviorOf(node);
 
     List<HInstruction> inputs = <HInstruction>[];
     addGenericSendArgumentsToList(link.tail.tail, inputs);
 
+    if (nativeBehavior.codeTemplate.positionalArgumentCount != inputs.length) {
+      compiler.reportError(
+          node, MessageKind.GENERIC,
+          {'text':
+            'Mismatch between number of placeholders'
+            ' and number of arguments.'});
+      stack.add(graph.addConstantNull(compiler));  // Result expected on stack.
+      return;
+    }
+
     TypeMask ssaType =
         TypeMaskFactory.fromNativeBehavior(nativeBehavior, compiler);
 
     SourceInformation sourceInformation =
         sourceInformationBuilder.buildCall(node, node.argumentsNode);
     if (nativeBehavior.codeTemplate.isExpression) {
       push(new HForeignCode(
           nativeBehavior.codeTemplate, ssaType, inputs,
           effects: nativeBehavior.sideEffects,
           nativeBehavior: nativeBehavior)
@@ skipping 4878 matching lines @@
     if (unaliased is TypedefType) throw 'unable to unalias $type';
     unaliased.accept(this, builder);
   }
 
   void visitDynamicType(DynamicType type, SsaBuilder builder) {
     JavaScriptBackend backend = builder.compiler.backend;
     ClassElement cls = backend.findHelper('DynamicRuntimeType');
     builder.push(new HDynamicType(type, new TypeMask.exact(cls, classWorld)));
   }
 }