Check that non-abstract classes implement all methods.

dart/sdk/lib/_internal/compiler/implementation/warnings.dart

Index: dart/sdk/lib/_internal/compiler/implementation/warnings.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/warnings.dart b/dart/sdk/lib/_internal/compiler/implementation/warnings.dart
index dc92997d8578b51858a56a47171f6261cc4903c2..2d711c3e41fa5d35aa6b911e43d6648e935944eb 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/warnings.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/warnings.dart
@@ -1360,6 +1360,41 @@ main() {}
       howToFix: "Consider deleting it.",
       examples: const ["deadCode() {} main() {}"]);
 
+  static const MessageKind UNIMPLEMENTED_METHOD = const MessageKind(
+      "Warning: '#{class_name}' doesn't implement '#{member_name}'.",
+      howToFix: "Try adding an implementation of '#{member_name}'.",
+      examples: const ["""
+abstract class I {
+  m();
+}
+
+class C implements I {}
+
+class D implements I {
+  m() {}
+}
+
+main() {
+ new D().m();
+ new C();
+}
+""", """
+abstract class I {
+  m();
+}
+
+class C extends I {}
+
+class D extends I {
+  m() {}
+}
+
+main() {
+ new D().m();
+ new C();
+}
+"""]);
+
   static const MessageKind COMPILER_CRASHED = const MessageKind(
       "Error: The compiler crashed when compiling this element.");