fixes #173, ability to use native JS indexers

lib/src/codegen/js_codegen.dart

Index: lib/src/codegen/js_codegen.dart
diff --git a/lib/src/codegen/js_codegen.dart b/lib/src/codegen/js_codegen.dart
index 95fcf811ffe8cdd8a3dd926da67403e1f569c24b..d3f5b351cd2f6efbdfb47d019447cfeb084e3dfd 100644
--- a/lib/src/codegen/js_codegen.dart
+++ b/lib/src/codegen/js_codegen.dart
@@ -109,7 +109,7 @@ class JSCodegenVisitor extends GeneralizingAstVisitor with ConversionVisitor {
     if (unit.directives.isNotEmpty) {
       var libraryDir = unit.directives.first;
       if (libraryDir is LibraryDirective) {
-        var jsName = getAnnotationValue(libraryDir, _isJsNameAnnotation);
+        var jsName = findAnnotation(libraryDir.element, _isJsNameAnnotation);
         jsDefaultValue = getConstantField(jsName, 'name', types.stringType);
       }
     }
@@ -328,7 +328,7 @@ class JSCodegenVisitor extends GeneralizingAstVisitor with ConversionVisitor {
     // If we've already emitted this class, skip it.
     var classElem = node.element;
     var type = classElem.type;
-    var jsName = getAnnotationValue(node, _isJsNameAnnotation);
+    var jsName = findAnnotation(classElem, _isJsNameAnnotation);
 
     if (jsName != null) return _emitJsType(node.name.name, jsName);
 
@@ -349,7 +349,7 @@ class JSCodegenVisitor extends GeneralizingAstVisitor with ConversionVisitor {
         _classHeritage(classElem), _emitClassMethods(node, ctors, fields));
 
     String jsPeerName;
-    var jsPeer = getAnnotationValue(node, _isJsPeerInterface);
+    var jsPeer = findAnnotation(classElem, _isJsPeerInterface);
     if (jsPeer != null) {
       jsPeerName = getConstantField(jsPeer, 'name', types.stringType);
     }
@@ -474,7 +474,7 @@ class JSCodegenVisitor extends GeneralizingAstVisitor with ConversionVisitor {
       jsMethods.add(_emitImplicitConstructor(node, fields));
     }
 
-    bool hasJsPeer = getAnnotationValue(node, _isJsPeerInterface) != null;
+    bool hasJsPeer = findAnnotation(element, _isJsPeerInterface) != null;
 
     bool hasIterator = false;
     for (var m in node.members) {
@@ -1404,7 +1404,12 @@ class JSCodegenVisitor extends GeneralizingAstVisitor with ConversionVisitor {
 
   JS.Expression _emitSet(Expression lhs, Expression rhs) {
     if (lhs is IndexExpression) {
-      return _emitSend(_getTarget(lhs), '[]=', [lhs.index, rhs]);
+      var target = _getTarget(lhs);
+      if (_useNativeJsIndexer(target.staticType)) {
+        return js.call(
+            '#[#] = #', [_visit(target), _visit(lhs.index), _visit(rhs)]);
+      }
+      return _emitSend(target, '[]=', [lhs.index, rhs]);
     }
 
     Expression target = null;
@@ -2195,9 +2200,18 @@ class JSCodegenVisitor extends GeneralizingAstVisitor with ConversionVisitor {
 
   @override
   visitIndexExpression(IndexExpression node) {
-    return _emitSend(_getTarget(node), '[]', [node.index]);
+    var target = _getTarget(node);
+    if (_useNativeJsIndexer(target.staticType)) {
+      return new JS.PropertyAccess(_visit(target), _visit(node.index));
+    }
+    return _emitSend(target, '[]', [node.index]);
   }
 
+  // TODO(jmesserly): ideally we'd check the method and see if it is marked
+  // `external`, but that doesn't work because it isn't in the element model.
+  bool _useNativeJsIndexer(DartType type) =>
+      findAnnotation(type.element, _isJsNameAnnotation) != null;
+
   /// Gets the target of a [PropertyAccess] or [IndexExpression].
   /// Those two nodes are special because they're both allowed on left side of
   /// an assignment expression and cascades.