Implement override checks.

dart/lib/compiler/implementation/resolver.dart

Index: dart/lib/compiler/implementation/resolver.dart
diff --git a/dart/lib/compiler/implementation/resolver.dart b/dart/lib/compiler/implementation/resolver.dart
index 3ab84c7dc809e0ce33018a5d292fe93d2917124b..94faf24a05a986c73eac58ed45b7b4308deaa6cf 100644
--- a/dart/lib/compiler/implementation/resolver.dart
+++ b/dart/lib/compiler/implementation/resolver.dart
@@ -229,6 +229,50 @@ class ResolverTask extends CompilerTask {
         new ClassResolverVisitor(compiler, element.getLibrary(), element);
       visitor.visit(tree);
       element.isResolved = true;
+
+      while (!toResolve.isEmpty()) {
+        ClassElement classElement = toResolve.removeFirst();
+        classElement.ensureResolved(compiler);
+      }
+
+      checkOverrides(element);
+    });
+  }
+
+  checkOverrides(ClassElement cls) {
+    if (cls === compiler.objectClass) return;
+    cls.forEachMember((holder, member) {
+      Element superMember = cls.lookupSuperMember(member.name);
+      if (superMember !== null) {
+        if (member.modifiers.isStatic()) {
+          compiler.cancel('Static members cannot override', element: member);
+        } else {
+          FunctionElement superFunction = superMember.asFunctionElement();
+          FunctionElement function = member.asFunctionElement();
+          if (superFunction === null || superFunction.isAccessor()) {
+            // Field or accessor in super.
+            if (function !== null && !function.isAccessor()) {
+              // But a plain method in this class.
+              compiler.cancel('Cannot override field with method',
+                              element: member);
+            }
+          } else {
+            // Instance method in super.
+            if (function === null || function.isAccessor()) {
+              // But a field in this class.

[karlklose, 2012/06/20 13:51:31]

... or accessor ...?

[ahe, 2012/06/22 12:50:33]

Done.

+              compiler.cancel('Cannot override method with field',
+                              element: member);
+            } else {
+              // Both are plain instance methods.
+              if (superFunction.requiredParameterCount(compiler) !=
+                  function.requiredParameterCount(compiler)) {
+                compiler.cancel('Bad override', element: member);
+              }
+              // TODO(ahe): Check optional parameters.
+            }
+          }
+        }
+      }
     });
   }