Create local alias for super class in --closure mode (issue #312)

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 88215c42ea58829381a016fc23d2cd62657d85fc..2b3fddff52f27b3870c7802ab85220cabf934bfb 100644
--- a/lib/src/codegen/js_codegen.dart
+++ b/lib/src/codegen/js_codegen.dart
@@ -379,10 +379,11 @@ class JSCodegenVisitor extends GeneralizingAstVisitor with ClosureAnnotator {
       }
     }
 
-    var classDecl = new JS.ClassDeclaration(new JS.ClassExpression(
-        new JS.Identifier(element.name), _classHeritage(element), body));
+    var classExpr = new JS.ClassExpression(
+        new JS.Identifier(element.name), _classHeritage(element), body);
 
-    return _finishClassDef(element.type, classDecl);
+    return _finishClassDef(
+        element.type, _emitClassHeritageWorkaround(classExpr));
   }
 
   JS.Statement _emitJsType(String dartClassName, DartObject jsName) {
@@ -674,6 +675,27 @@ class JSCodegenVisitor extends GeneralizingAstVisitor with ClosureAnnotator {
     }
   }
 
+  _isQualifiedPath(JS.Expression node) =>
+      node is JS.Identifier ||
+      node is JS.PropertyAccess &&
+          _isQualifiedPath(node.receiver) &&
+          node.selector is JS.LiteralString;
+
+  /// Workaround for Closure: super classes must be qualified paths.
+  JS.Statement _emitClassHeritageWorkaround(JS.ClassExpression cls) {
+    if (options.closure &&
+        cls.heritage != null &&
+        !_isQualifiedPath(cls.heritage)) {
+      var superVar = new JS.TemporaryId(cls.name.name + r'$super');
+      return _statement([
+        js.statement('const # = #;', [superVar, cls.heritage]),
+        new JS.ClassDeclaration(
+            new JS.ClassExpression(cls.name, superVar, cls.methods))
+      ]);
+    }
+    return new JS.ClassDeclaration(cls);
+  }
+
   /// Emit class members that need to come after the class declaration, such
   /// as static fields. See [_emitClassMethods] for things that are emitted
   /// inside the ES6 `class { ... }` node.
@@ -702,7 +724,7 @@ class JSCodegenVisitor extends GeneralizingAstVisitor with ClosureAnnotator {
       }
     }
 
-    body.add(new JS.ClassDeclaration(cls));
+    body.add(_emitClassHeritageWorkaround(cls));
 
     // TODO(jmesserly): we should really just extend native Array.
     if (jsPeerName != null && classElem.typeParameters.isNotEmpty) {