Emit compile-time errors for conflicting modifiers.

sdk/lib/_internal/compiler/implementation/resolution/members.dart

Index: sdk/lib/_internal/compiler/implementation/resolution/members.dart
diff --git a/sdk/lib/_internal/compiler/implementation/resolution/members.dart b/sdk/lib/_internal/compiler/implementation/resolution/members.dart
index 4135d0cc3585607fda7a57ae98f58db2ebdae025..d69635e889e0642f272536c869eee16bccee7833 100644
--- a/sdk/lib/_internal/compiler/implementation/resolution/members.dart
+++ b/sdk/lib/_internal/compiler/implementation/resolution/members.dart
@@ -2727,6 +2727,27 @@ class ResolverVisitor extends MappingVisitor<Element> {
     } else {
       visitor.variables.type = compiler.types.dynamicType;
     }
+
+    Modifiers modifiers = node.modifiers;
+    void reportExtraModifier(String modifier) {
+      Node modifierNode;
+      for (var nodes = modifiers.nodes; !nodes.isEmpty; nodes = nodes.tail) {
+        if (modifier == nodes.head.asIdentifier().source.stringValue) {
+          modifierNode = nodes.head;
+          break;
+        }
+      }
+      assert(modifierNode != null);
+      compiler.reportError(modifierNode, MessageKind.EXTRANEOUS_MODIFIER,
+          {'modifier': modifier});
+    }
+    if (modifiers.isFinal() && (modifiers.isConst() || modifiers.isVar())) {
+      reportExtraModifier('final');
+    }
+    if (modifiers.isVar() && (modifiers.isConst() || node.type != null)) {
+      reportExtraModifier('var');
+    }
+
     visitor.visit(node.definitions);
   }