Fix some warnings.

lib/compiler/implementation/elements/elements.dart

Index: lib/compiler/implementation/elements/elements.dart
diff --git a/lib/compiler/implementation/elements/elements.dart b/lib/compiler/implementation/elements/elements.dart
index 390d775f5e8968ba4bcdcb888eff0d33fd086f3a..20d62e8ce227eca2551fee72566eb7d4f109b5f8 100644
--- a/lib/compiler/implementation/elements/elements.dart
+++ b/lib/compiler/implementation/elements/elements.dart
@@ -1304,7 +1304,7 @@ abstract class TypeDeclarationElement implements Element {
   }
 }
 
-class ClassElement extends ScopeContainerElement
+abstract class ClassElement extends ScopeContainerElement
     implements TypeDeclarationElement {
   final int id;
   InterfaceType type;
@@ -1330,6 +1330,8 @@ class ClassElement extends ScopeContainerElement
       resolutionState = initialState,
       super(name, ElementKind.CLASS, enclosing);
 
+  abstract ClassNode parseNode(Compiler compiler);
+
   InterfaceType computeType(compiler) {
     if (type == null) {
       if (origin === null) {